I would ask the question the other way: How does replacing the roller wheels invalidate the warranty? Where's the reasonableness in that?
It's like putting mag wheels on a car and that doesn't crap out the warranty.
The Brompton wheels are designed in conjunction with their support pieces. Aftermarket wheels may result in forces on the support pieces for which the support pieces are not designed.
And, in the USA, putting after-market wheels on a car that don't conform to the manufacturer's specifications for wheels for that vehicle may void the manufacturer's warranty, especially on suspension parts that are not designed for the forces generated by the non-conforming wheels.
Let's be clear. Making a potentially warranty-voiding modification to any bicycle, and then undoing that modification in conjunction with making a warranty claim on the bicycle without disclosing that the bicycle had been modified. is, in almost every instance, dishonest.
-HANK RYAN-
Norman, Oklahoma USA