The refusal of a visa could be a pressure tactic by the UK to make the Sharifs agree to certain political conditions to renew the VISA, that PML must enact "on" Pakistan if they ever come to power again ?! There is more to this, than meets the eye.
Are the USA/UK/Quad combo looking to dispose of the PTI through whatever means they "can" and then if PML manage to win the next sdet of political musical chairs, have a compliant goverment in place with agreed actions. ie hedging their bets.
PTI has a adopted a Pakistan first and Pro-Pakistan approach to their decisions, especially foreign policy and i can imagine that the west is tired of this and wants a complaint Pakistan that yields to both the west and India ?
(that is my conspiracy theory for today).