Given that Swytch supply a Brompton specific kit, I'd assume that they have supplied an 8mm axle to suit - unlike the normal kit I have, where the axle is 10mm and needs a small amount of filing the normal 9.5mm dropouts width.
The other problem, which is what I think is happening here, is that fitting the lock washer (essential to keep the motor from rotating in the dropout) has meant the axle won't seat as far up as it should. This requires filing out of the dropout depth.
HTH