@wejammin we have just come back from a week in Cornwall. The weather forecast looked DIRE (cold and rainy every day, no sun). I almost said I didn't want to go.
The reality was it was fine. Not hot, but warm and sunny with clouds in the sky for most of the week. We had a few rainstorms but they mostly passed quickly and then the sun would pop back out again. We went to the beach, swam in the sea, had bbqs, day trips etc.
I actually wished I'd packed more summery clothes!
So try not to stress, hopefully it'll be the same for you - the south in general hasn't been as bad as the rest of the UK by the sounds of it.