Well, okay, this is a twist. This decision was taken yesterday.
Final solution for British tanks changes (this overrides any previous info, including the earlier Evilly post):
Tier 10 medium tank will be Action X Centurion (read about it more here) with 105mm L7A1 (it had a 20pdr in real life, but both weapons were interchangeable in its mounts). Its features will be very strong turret (220mm to sloped 150mm) plus very good depression. It is not yet decided how the FV4202 to Action X change will look.
FV4202 will be changed to tier 8 premium tank and brought to its historical parameters (upper frontal plate 50mm, lower frontal plate 75mm, turret from 200mm to 60mm (the sloped part), 20pdr gun). It is not decided yet whether it will be given for free.
Tier 10 British heavy tank will be the Chieftain Mk.2