All balance aside, the biggest downside of the UKF in my opnion, is how linear and predictable they are to play with and against.
Linear teching aside I think their greatest downfall in this department, is that they have literally no opening game alternative to infantry sections. They are the only faction who does not have neither t0 or instantly buildable doctrinal infantry, nor non-doctrinal infantry alternatives, which means that 2-5 squads of Sections is almost guaranteed every single match.
This makes it literally impossible to throw any real curve balls in the opening game other than some flavor from a UC or Vickers.
In retrospect, I think the UK could have benefited a lot from an alternative opening infantry unit back when they were being developed.
Well hang on a minute, USF has no alternative to Riflemen, that's the faction that's the most predictable. UKF at least has UCs and MGs, actual fighting units. USF only gets Riflemen for actual fighting units when the game starts.