It must have required more than a little courage for him to pose beside it. After all, the studied insult of his long-time sparring partner, Sir Terence Conran, that Sir Roy should be "stuffed and exhibited in a case at the V&A", must have been ringing in his ears. "Well," says Sir Roy, with an air of suppressed indignation, "he's achieved his end."
It is nine in the morning and he is wearing a Nicole Farhi frock-coat, an Italian-style belt with decorative silver links from Jermyn Street, and a Versace baroque black-tie. The coat had cost him pounds 350. "It's the only frock-coat I've got," he says, with mingled apology and regret: "Wonderful design: I tried it on and thought, `I do like that look.' Luckily, I can get into ready-mades.
"I once had a frock-coat in brown velvet with black lapels. I used to wear it as the equivalent of a dinner jacket in the late Sixties and early Seventies. David Bailey photographed me in it with lots of cigarette smoke around."
Sir Roy is 61 now, and has forsaken the King's Road and Carnaby Street, where in the mid-Sixties he could dress in Regency-style damask for as little as pounds 25. Now he is often to be found in Harvey Nichols' contemporary menswear basement in Knightsbridge, where there are "nice young assistants who know me"."I'm not aware of my age," he says. "I certainly don't feel I've got to become an old man in a grey coat. You don't dress for age any more, you dress for style."
The brown cotton-velvet jacket and waistcoat under the spotlights at the V&A's exhibition, The Cutting Edge: 50 Years of British Fashion, was made for him in 1968 by the now defunct Just Men shop in the King's Road. He had bought the blue lining and the ribbed silk-ribbon edging for the flapping lapels at Harrods. The brown silk tie with double Windsor knot came from Turnbull and Asser.
"It is a reflection of the huge explosion of dandyism between 1965 and the oil crisis of 1974," he says, the dandy suddenly becoming donnish. "By 1975, you would have been either a freak or a member of a Midlands Rotary Club to wear anything like that.
"The trousers are pre-flare. Flares came in in 1970 and went out abruptly in 1974 when the economy collapsed. Everything became narrow. Three-inch- high collars shrunk to an inch. There was a complete fashion revolution reflecting the huge economic change. Dandyism," he insists, "was an English tradition. It was only in the 19th century that clothes became dull - and really only in the late Sixties and early Seventies that dandyism came back. You never see a caftan at the opera any more. But I'm glad there are signs of it returning once more. When I pass London clubs at night and see queues of men dressed as women, I think they make my dress look terribly feeble."
When things suddenly shrank, he was aged 39 and had switched from being the stylish golden-boy director of the National Portrait Gallery to embroil himself with the mandarins of the V&A. Within two years, the Labour government had demanded a loss of more than 100 of the V&A's staff. In whichever direction he faced, the knives were out.
"All that era of doing the social scene - openings, parties, dinners - became part of a period that has now gone." King's Road and Carnaby Street might have been on their uppers, but not he. He turned to Versace, Farhi, Paul Smith.
If there is one item of dress that symbolises the progress of the boy born on the wrong side of the park to Social Lion of 1968, as the journalist Ann Scott James dubbed him, it is those tartan trousers of the V&A evening dress.
"I stayed that summer with Hugh and Antonia Fraser in their house on the island of Eilan Aigas in Inverness. There was a shop nearby that sold tartans with natural dies. I thought this Campbell hunting tartan so beautiful that I had to have it made up on the King's Road."
Why the compulsion to dress up? "I have never minded causing a bit of a stir by what I wear. Why not? Everybody in the late Sixties - the great peacock era - was drawing attention to themselves."
Then, more reflectively: "My childhood was not a happy one. I was crippled by shyness and retreated into a fantasy of the past. I was obsessed with it. I've recently realised why I did it: it was to escape the present."
He sketched the costumes in the V&A and pestered his teacher with costume designs for school plays. He wanted to become a theatrical designer. "Being the son of an impoverished commercial traveller, I came out of grammar school as a meritocrat, without status from birth or wealth. So I dressed for effect. I knew perfectly well what I was doing - I was fashioning an identity for myself as part of a new group in society. People like Cecil Beaton were models for it. The dandyism was partly a mask, partly a passport to high Bohemia. There was a sort of defiance about it."
He quotes the Victorian writer CF Forbes: "The knowledge of being well- dressed gives a feeling of inner tranquillity that religion is powerless to bestow." Except that Sir Roy misquotes "confidence" for "inner tranquillity".
First steps in dandyism came when a local tailor in Palmer's Green began making Edwardian-style coloured waistcoats - "the first daring style that a man could have", Sir Roy explains. That was in the mid-Fifties. He sinks into a reverie: "A wine-coloured silk waistcoat flecked with gold ... a pale beige overcoat with brown velvet collar."
As director of the National Portrait Gallery, he did make an attempt to disguise himself as a civil servant. That lasted three months. "I thought, `to hell with it, they've appointed me for what I am'."
Back came the velvet and silk. No matter the journalists' sarcastic comparisons with Lord Byron, no matter the side-cannon from the old guard, such as Field-Marshal Sir Gerald Templer, an NPG trustee, who told him to get his hair cut: or - a more recent mishap - discovering that a maid at the British Embassy in Madrid had ironed flat the tastefully crumpled brown linen suit that his friend Versace had given him.
What counted was that, in 1968, Beatrix Miller, editor of Vogue, sent Cecil Beaton to photograph him - looking like a frightened schoolboy against a background of Elizabethan grandees in ruffs, in a painting. "My career took off," he says.
Beaton invited him to lunch with the Queen Mother. She opened the NPG exhibition of Beaton's photography. Vogue had earlier held an opening bash at the Gallery attended by what seemed to be the whole of Swinging London. The exhibition was a sensation. "I never looked back."
No doubt there will be scholarly musings on dandyism and the making of his reputation in his forthcoming history of British culture, companion to The Story of Britain, published last year, which has so far sold 40,000 copies. But before that, the Roy Strong diaries: running battles at the V&A revealed - plus a night-by-night what-I-wore guide, not forgetting the quizzing glass.
He and his wife, Julia Trevelyan Oman, now tend the formal garden of their home in Herefordshire (he has written five gardening books), pausing every five years to bag up more of Sir Roy's cast-off clothes in the loft.
This ritual is regarded with envy by Avril Hart, the V&A's assistant curator of dress. She collared him as he revisited his brown evening dress at the V&A to tell him that what she really wanted was his Eighties white Versace suit.
`The Cutting Edge: 50 Years of British Fashion' at the V&A, London SW7 to 27 July.Reuse content