But now this small group has been joined by the private buyer. While two sects are showing interest in Pierrepont School near Farnham in Surrey, an enormous complex including a technology centre and rifle range, so an international businessman is considering buying St Michael's, a former girls' boarding school near Petworth in Sussex.
Two years ago the mansion market was almost dead. Any buyer at any price was more than welcome. Today, hotels and conference and training centres, which closed at a swift rate during the recession, are still non-starters. But other areas of the market, particularly in the South, are picking up.
Some of Britain's finest buildings are again being transformed into nursing homes and company leisure centres, as well as being taken up by religious groups. Two such groups bought the beautiful Hampton Court estate in Herefordshire and Clarendon Park in Bedfordshire. Also popular once more is the residential split, whereby a mansion is converted into houses and apartments.
St Michael's is one of the most saleable of the many mansions buyers can choose from. It is within striking distance of London and its airports, it has a glorious setting with a 20-acre lake in 150 acres of grounds, and the main house is a splendid Grade I building. The ugly modern additions - gyms, classrooms, and science labs - could probably be bulldozed away with the planners' blessing.
Philip Blanchard of John D Wood in Winchester has a developer client with a good track record who is one of many interested in buying St Michael's, valued at pounds 2- 2.5m by Savills.
The old Hawtreys prep school in Tottenham House on the Savernake estate in Wiltshire, is also attracting interest. Again, the setting and the principal building are particularly fine. John Inge, who is selling it for Knight Frank & Rutley, recently spent a day there with a potential buyer. 'We sat on the steps underneath the Palladian pallisade and the only sound was of hooting owls,' he said.
'These marvellous buildings do excite a certain appreciation of fine architecture,' he added. 'With so much business conducted by fax and computer these days there are businessmen who can arrange to live and work in a house like this.'
One man who has already done it is Nigel Tuersley, the owner of Wardour Castle, another Grade I Palladian mansion in Wiltshire. He bought it for under pounds 1m nearly three years ago, when the market was at rock bottom.
Mr Tuersley has created a home around the vast central rotunda which rises 60 feet to a skylit dome. He is converting the top floor of the house into three large apartments, which will be rented for about pounds 20,000 a year. The place is crawling with craftsmen.
Outside, the gardens landscaped by Capability Brown are being restored to their former splendour. He is replanting magnolias and acres of wild flowers, and restoring the walled vegetable garden.
'Nigel Tuersely went about Wardour in a very clever way,' said Philip Blanchard of John D Wood, which is handling the rentals. 'When he submitted plans for possible new-build homes, his architect was Quinlan Terry. He is totally committed to Wardour and has worked out a way to spend the rest of his life there.'
The market for apartments in mansions is buoyant at the moment. 'The reason the developers have come back is because the properties are selling,' said Nick Sweeney of Savills.
But the location has to make commuting or weekending a possibility, and both the house and grounds must be attractive. 'Just because it is a big pile doesn't mean it is very valuable,' Mr Sweeney added. 'A mansion may be Grade I, but it may also be in the wrong place and need huge amounts spent on maintenance.'
This is exactly the problem with many of the mansions on the market in the North. While Savills is asking more than pounds 2m for somewhere like St Michael's, it is asking only pounds 275,000 for Moresby Hall near Whitehaven in Cumbria.
The 16th-century Grade I house was used by an aluminium company for corporate entertaining. Now, without interest from British Nuclear Fuels - one of the few fat purses in the region - it is hard to know who will buy it.
'The difficulty at the moment is finding someone who will put money up front and take a risk on getting it back,' said Ben Hudson, who is selling it for Savills in York. 'There is still a lack of confidence in the market up here.' Mr Hudson estimates properties such as this have halved in value in the past few years. Savills has reduced the price of Rusland Hall in Cumbria from pounds 850,000 to pounds 475,000 and is only now starting to receive offers.
With no market for residential split-ups, most mansions have to go to a business or a private individual. Jonathan Major of Strutt & Parker in Chester sold the vast Llantysilio Hall in Wales to a private buyer for just pounds 300,000. For Broughton Hall in more accessible Staffordshire, a private buyer paid pounds 750,000.
As Ben Hudson said: 'You can buy a lot of bricks and mortar very cheaply up here.'