LWaB, did you check the actual size of your seatpost? Just in case this is an unfortunate clash of tolerances, with the post being slightly undersized?
The glue is probably the unknown in the equation. Brompton should be perfectly capable of making a seat tube to an accurate ID, since everyone else can (the "frame variation" excuse is BS from them). However, if you're going to stick a shim in with glue, it all becomes a bit random, especially if you are cleaning old glue out and putting new glue on top.
A glueless insert, secured by a lug at its lower end to stop it popping out with the seatpost, would seem to be a sensible way forward. But Brompton don't move forward - no need to, when they can sell them faster than they can make them.