If I went brompton then realistically I’m looking at the S2L due to budget. You seem to get so much more for your money from the other brands.
At the moment my commute is a doddle and could be done on a singlespeed no problem. Flat across Windsor, on the train, then flat across the city. But a move to Bristol is in the offing and I don’t want the bike rendered useless by the hills as it would become both commuting and general getting about town transport.
The sensible option would seem to be a 7sp 2011 Dahon Mu with the nexus hub for under £500, or the new Tern equivalent with 8sp deraileur gears for under £500. Both have the 20″ wheels, which appeal, but neither fold as small as the Brompton, though the fold looks to be pretty quick.