The regs allow for this in table 4D5.
If the cable is in a stud wall and touching the inner wall surface (dissipates heat), then 2.5mm can carry 21amps, so fine for rings and 20amp radials.
If it is run in the middle of thermal insulation, then the rating is 13.5amps, so not large enough for a ring or a radial. You would need to increase to 4mm. This has a rating of 17.5amps run in the middle of insulation, so fine for a ring, but still not OK for a 20amp radial.