WMD / Jerricho...
Most pilots will obviously try and ask before deviating for weather... as you say.
However, one of priciples of flying is:
Aviate Navigate Communicate
or its more "PC" version
Plane Path People!
UK ATC frequencies are the busiest I have met. It may not appear so to you, since you are working at 150%, but it can take a few minutes to get a call in sometimes. This is fairly unique to the UK, and IMHO is the most dangerous part of our ATC system. It is also not uncommon to try and make a call, say 4+ times, and each time get stepped on / not acknowledged.
As a general "rule", I will not take an aircraft into something painting "red" on the radar. If I have failed to get ATC clearance, either because I left it too late, or I could not get a word in, the sorry - it comes down to priorities. It comes down to the probablilites of a problem due to the weather, and red returns could mean loss of the aircraft and occupants, as could (would!) a mid air. And of course, CBs "develop", and what was green 2 minutes ago has just started painting red...
That said, 99% of the time we get in the call, you go out of your way to help (far more so than other ATC agencies), and god knows how you cope with 15 aircraft all wanting to deviate.
Keep up the good work!
NoD