If you look on EBay, you can find a Merc--a Taiwanese Brompton knockoff--for 330 British pounds. At least on paper, it is a better buy with all of the accessories. Ignoring the accessories, the bikes differ by the (1) internal hub brand--Sturmey Archer vs. SRAM--, (2) type of shifter--grip vs. thumb shifter--, (3) frame material--alloy vs. cromoly. Look for the EBay userid 123maddy.

My wife and I have been comparing an older Brompton and a lightly used Merc. We have been using the Brompton for about a month and the Merc a little more than a week. So we are unable to make any big statements on the Merc's reliability. I have tinkered with the bike a bit and so far the Merc compares well.

You can find several postings in the Folding Bikes section on Bromptons and Mercs. There are Merc owners that give their impressions over a few months. A big plus for the Brompton is that there is a relatively large support network for the bike. That is, there are a lot of shops that carry the Brompton and the manufacturer stands by its product. You should factor that value into your decision.