Don't Revell do a London black taxi cab in 1/24th? That would be perfect. Also, you hardly ever see a stretch of UK road that has purely Brit cars on it unless there's a one-make rally going on, so some foreign-but-ordinary cars would fit right in. VW Beatles were ubiquitous in their day, as were the VW minibus, The VW Golf, the Citroen 2CV and the Fiat 500, all of which have been kitted. Revell/ESCI did a Ford Escort and that's as 'British' a car as anything (made here and hugely popular in it's day).
Just remember that cheap, ordinary family cars are WAY more common that exotic sports cars or bourgemobiles.
It really depends on which era you're going for. The most modern car on the board will set the date, and the older a car is, the less likely it is to be there.