Handlebar height may be too low, As you are in the UK , Try to contact
http://foldsoc.co.uk/
There is a Brompton modifying ingenious guy there Steve Parry Ive only seen his projects online
from the fold soc site I copied his phone number , perhaps he can modify a telescopic handle bar stem, to come up to meet your comfortable height .
Fitting a Modolo Dumbo Bar on is another possibility, though It would be fiddly as It would not fold any better than the P bar as the height is part of the front fold.. It would hit the ground when folded.
so You would need to re adjust the bar setup each time.
I saw a picture of the tandem he made of 2 brompton main sections , that it folds up and will stand on end in the closet is brilliant. picture was taken at a prior years Brompton world championships race at the Churchill's old place..
phone listed as 01934 516158 he has no other contacts , a mailing address would be handy , perhaps whyen you get in touch with him have him suggest a snail mail method to contact for the stateside.
a rear assembly modified to fit my Rohloff CC EX into would be excellent , particularly if fatter Greenspeed scorcher TR Tire clearances are offered in the mix. and perhaps a front fork .. but I'm in Oregon..
I can understand avoiding a website..