Graham,
I assume you are talking about the " dead rubber" at the side of the tread area
It's because you are going quick !!! That's the rubber you've worn off the soft treads, it gets hot and rolls to towards the side wall. You need to get a rasp and remove it from the tyre before your next run, as it is no longer of any use. You should clean it off after every run, it will help your handling, also wash the front tyres before each run and get a pair of tyre wraps so you can drive to the line on clean tyres, that really makes a big difference for the first turn. I didn't do that until Doune one year and I nearly put it into the inside barrier on the first turn as the front end just gripped so much more that I was used to.
Yes, some of our tracks are bumpy, but Craigantlet isn't and neither is Cairncastle, if the BHC guys can run their Pilbeams and Goulds without problems then they are OK.
The Chanel Islands are dear because of the all the ferry trips. It used to cost me about £500 to do Harewood, but that included two night in the Harewood Arms hotel, our event are only one day, so you don't have as much expense. I always enjoyed doing the GB hills for a bit of variety, it's good to go somewhere different every now and again. I could never understand folk who just did the Midland Triangle and never ventured further afield.