Aftermarket skate wheels do work well (as already said above), but there can be issues using them with the standard Brompton rack (I only have experience of the Advance rack). One of my second-hand P-Lines came with an extremely light weight triangular shaped rack and massive skate wheels. There were no heel-strike issues with this combo, but I wanted compatibility with my other P-Lines so I replaced the rack and then found issues with heel-strike and the skate wheels.
Now I am using the advance rack on three P-Lines with G-Line roller wheels. Note that the G-Line uses 70mm for the frame and 80mm for the rack. This combo rolls really well and feels sturdy, so I would recommend it. Only issue is the cost, significantly more than aftermarket.