Originally Posted by
fietsbob
P & M are putting the bar tops at a similar height,
No, they are not and never have been.
This ist the old M-bar which is lower than the P. The OP uses the new M-bar which is lower, thus delivering even less height. The P stem and the old M stem are about the same height. The new, current M stem is higher than the P and the old M, and the new M bars are lower, overall height is the same as the old M.
The OP is now between M and S heightwise with his bike setup, based on a P stem with the new M bar.
Using the original P cable length on a bar that is much lower obviously like the OP does works but can create a number of issues, resulting from the cables being too long. When folding or unfolding the cables may get cought at the stem hinge, resulting in issues when folding. The cable gatherer will possibly be too low now, resulting in possible issues with the fold as well like i.e. rubbing cables and paint damages as well as (with some bad luck) the chain coming off during the fold occasionally. There is a reason why the different bars have cables of different lengths. Longterm, the complexity of the Brompton will probably win. Sooner or later.