Can anyone tell me why the UK weather has been set in an anticyclonic
pattern, virtually non-stop, since the beginning of 2003?

Having replied to your first respondant, I ought to reply to your question!
Airflow does sometimes get into a long term rut, and there is an element of it
just balancing out the cyclonic period 2000 to 2002 (I agree with you that this
is not a reason for complacency). However, this summer was not especially
anticyclonic - though the last few weeks obviously have been. After a long dry
spell, the dryness of the ground forces a positive feedback on the climate
system, discouraging rain, amplifying the original 'dry' signal. This is
probably how the dry spell seems to gain momentum. I'll leave it to others to
suggest when it will change, though I see from the radar it already has done
further north today!

Julian Mayes, West Molesey, Surrey.