Just for remembrance, its possible to mount 20" ETRTO 406 wheels on any Birdy and there is now a Pacific Cycles Birdy model equipped from factory with 20" wheels,

The 18" ETRTO 355 wheel size (so only 6mm bigger than the 16" ETRTO 349 of the Brompton wheels) is now a drawback due to the limited choice of tires compared to the current wider choice of tires in ETRTO 349 (this is a relatively recent evolution, this wider choice of tires in ETRTO 349 didn't exist a couple of years ago).

Having all types of small wheels bikes and also big wheels bikes, for a road use, I am absolutely not convinced by the advantages of big wheels and on a folder, bigger wheels is definitely a drawback for the fold and for the weight.

The folding frame of the Brompton should not be a reason to reject it: the hinge system of the MK IV Brompton is fully reliable and last very long. Replacement of the hinge pin is almost never needed.