Christopher Maume: 'The British National party is right about one thing: the country is filling up'

I am not a fan of the BNP, honest. Nick Griffin, patron saint of the benighted and cross-eyed, is as about as high on my list of People To Hug Before I Die as Melanie Phillips or the Grimes twins are. By way of consolation, they all enjoy exalted placings in the list of People To Hang Before I Die. But the British National Party are right about one thing: the country is filling up. Bill Bryson could have called his love letter to Britain Notes from a Small Island With Too Many People, Especially in London.

The capital must have been hell in the 1930s, at its peak population of 8.5m. Now it's a million down on that – but most of them seem to want to get into all the same places as me at exactly the same time (not to mention catch the same train, shop in the same supermarket and hail the same cab). The museums in South Kensington are a half-term militarised zone; taking the family to the Thames Festival in the summer was to be caught up in a human river running parallel to the watery version, with no chance to fight the tide. Alan Johnson says he doesn't lie awake at night worrying about the prospect of a UK population of 70m; well, he should.

If Johnson is perceived as the bouncer who's rubbish on the door, I have a similar ongoing domestic scandal: Filter and Splat, goldfish acquired as replacements for a dead cat a few months ago, have been recently joined by Tiffany and Ruby, given as a birthday presents to my stepdaughter by friends who should have known better. As a vegetarian who doesn't really believe in pets and disagrees with keeping animals cooped up, I wasn't happy about the fish in the first place, but was persuaded that the tank we were buying was big enough for two of them. In fact, it's big enough for three, according to the manual; but not four. At the expense of family concord, arrangements are being made to rehouse the asylum seekers.

Piscine overcrowding causes stress ("Stress? In fish? For God's sake," my stepdaughter muttered). It makes disease more likely, depletes oxygen levels and ramps up the parasite quotient. Much like human overcrowding. Perhaps with the plight of Splat and Co in mind, there was a family visit to London Aquarium at the weekend (next to an attraction whose name I forget, but it's something like "The London Near-Death Experience", its punter-pulling strategy consisting of dressing up its operatives in ghoul gear and sending them out to scare children and pensioners).

Behind the glass, the aquarium was a mirror image of multicultural Britain, in which by and large we bump along together peacefully: weedy sea dragons sidled by big-bellied seahorses (those are their names – I'm not being rude), wrasses wriggled past rays, leopard sharks shot the breeze with turtles as big as my kitchen table. No stress there, then (and did you know that octopuses have blue blood and three hearts and are more intelligent than the average Premiership footballer?). On the mammalian side of the glass, however, the human factor prevailed, which wasn't good. All those big brains, so little thought.

It's not that the Aquarium people let too many in; I'm sure current consumer-density guidelines are adhered to. But it feels like too many, which I think is a function of the general decline in urban civility and the rise in urban stupidity, all fuelled by population overload. Signs and announcements such as "Flash photography can hurt the fishes' eyes" and "touching the rays is bad for their health" were explicit and unmistakable. But it was an epileptic's nightmare in there and a ray-tickler's dream. How thick are people (they were mostly natives, incidentally)? Outside, on the steps leading up to Westminster Bridge, an entire family, six or seven of them, were spread out across the full width eating lunch while bottlenecks built up above and below. Hand me my Kalashnikov, darling, and look the other way.

The depressing thing about all this is that apart from breeding less and forgetting about medical advances, the human race has no solution. Short of a souped-up swine flu or something similar wiping out a chunk of the world's population, we're all, as they say, doomed. Goodbye self-restraint, hello mass extinction.

John Walsh will return next week