Lets be realistic, the game cannot fit a 5th faction.
The first issue is the equipment already present in US tech tree, which cannot be taken away or transfered because that would anger many players.
A standalone British faction would suffer from low player count thus it would always match with US anyway so nothing would change other than people like me or Erika could no longer have mixed British and US line ups.
Not to mention that with the snail’s current pace, it would take 5 major updates (roughly 15 months) to add all these without anyother faction getting anything.
Lets not forget about BP either.
Already existing British BP weapons and soldiers cannot be removed or moved from US faction.
Britain will need its own BP weapons, vehicles and soldiers added to the rotation which will increase the wait time for all factions to get something out of BP.
Overall, nothing to gain but many headaches to suffer from this, not worth it.