When I spend good Friday riding 110 miles into a headwind, I'll think of you and be glad I only have one day of this.
I did the LEJOG (my second) with Amit last year - that's my bum up there in the picture. General (non-weather related) things I learned:
-take as little as possible (we stayed in B+Bs. I think I managed about 3kg of luggage on a rack pack)
-do a dry run carrying all your stuff (er bit late for that now!!!)
-eat lots throughout the day (long lunch stop + grazing suited us, but each to their own) and ASAP when the day is over, even if its just recovery chocolate milk before dinner.
-it really is true, after Devon and Cornwall everything else seems easy.
We rode nine days in mid May, and I spent all of the wettest April on record worrying about the weather. We actually started out in similar conditions to this - not this cold in May, but still unseasonalbly cold, 10/11C at Land's End with a bitter NE wind. By the time we got to Scotland it was 20C and sunny. By the time we got to North Scotland it was 27/28C!!!
The first days are going to be tough with current conditions, but just keep pressing on and it will get easier. And as the above illustrates, the weather in the UK will always change sooner or later. Even if we revert to weather fronts piling in off the Atlantic, you will be wet but you'll have a tailwind