You can't go to round cable since the ampacity of the round 2.5 mm2 cable is only 19A whereas the flat is 20A. The flatness increases the surface area somewhat.
This became an issue when they introduced the new requirement for at least 20A capacity per "leg" in the ring. The old currency carrying capacity table was based on round cables and would have ruled out rings on 2.5mm2 in some cases. Therefore, they retested the T&E until they got 20A.
Of course, you could also solve it by increasing the temperature rating of the cable.
