Some alternatives for you to consider.
Have a look at Mezzo folding bikes, In you budget range you should get a used one in good condition without any problems. If you are lucky you might be able to get a used Birdy for £300-£350. I would also consider some of the higher end Bfold bikes from Decathlon. I haven't seen any in person but having looked a photos of the higher end ones they seem to have good looking frames, components may not be so good.
If you want big wheels then a used Dahon Jack/Espresso should come in well with your budget, as will a Montage, especially if you go for the first generation all steel framed Biframe (in the UK they were branded Rudge Biframe).
If the idea of a Raleigh 20 does float your boat then I would advise a Dawes Kingpin instead. Very similar to the 20 but in my opinion a better frame and more importantly, the bottom bracket threads are industry standard (the Raleigh 20 had a different thread pitch).