EV – they have braze-ons the forks for the mounts. You could have this done. There are places that do this. The Kool-Stop (Salmon colored MTB) pads will help too. This is very important to get the salmon colored MTB pads. You have to juggle them a little getting them in there and due to the small diameter rim they have to be aligned perfectly but what a great improvement. We go from marginal Brompton brakes to at least adequate. By adequate I mean they will actually slow you when needed but if you are heavily loaded, touring and in the hills I would still exercise caution.

The upgraded brakes on the Merc GT6 will stop on a dime and give you change. EV, another upgrade that will help is possible better levers in general. I don’t know on the Merc GT3 what they have but if they are like the C model Bromptons they are pitiful and pathetic. The higher end Brommies levers are better than the C models but still not great.

EV- if you can’t find anything PM me and I’ll see what I can do to help. If need be I could ship you something as US mail to the UK isn’t that expensive. I have found that compared to EU prices it would be cheaper to buy here and then send there even after you pay postage on smaller items. For example a mirror here that was $15 was 20Euro. It would cost about $4 or so to mail it via airmail. Still works out to be a decent cost savings especially if you buy several parts at once that aren’t super heavy.

The beauty of the Merc is it starts out inexpensive so even if you do upgrades that net you a better bike than a Brommie you still have less money in it. FYI, you could drop your gearing if you want to so it will climb hills better and give up a little top speed. For me I'll gladly make that trade-off as if necessary I can try and pedal faster.