Brompton: where to find the higher supply of second-hand bikes in UK, or anywhere in Europe?
If you'll be in the Netherlands any time soon, many used bikes are sold via www.marktplaats.nl. There are also a lot of addvertisements from shops, including these to:

Advertisement 1

Advertsiement 2

The second one has some 2013 M6R Black bikes.
I don't have experience with these shops. I do often use marktplaats.nl.