while I don't fly in the UK, I will tell you this. by asking the question here on
pprune, you are staying ahead of the plane.
by the same reasoning, you should know before you takeoff that you will have to avoid wx. Advise each controller on initial contact that you will need deviations at pilots discretion and direct to the "X" vor or fix when able.
do this before you need it...that's staying ahead of the game.
I do agree that if you are truly unable to communicate and YOU need to
deviate, selecting 7700 is appropriate...and then you may do as you please to
handle the emergency you just declared. And if you truly can't communicate
you could monitor the assigned freq on one radio and attempt contact on
another freq (last or expected or 121.5).
In the USA, just prior to takeoff (airline, IFR, wx) I advised tower,, be advised
we will deviate as neccesary if we are unable to communitcate with departure
control or center. if they can't handle it...they could deny me takeoff clearance.