Any good quality oil will be fine. Just treat it like a series of mini track days and keep an eye on brake pads wear, tyre wear and change the fluids regularly. On the subject of tyres Toyo 888's are about the best road legal tyre for this sort of thing.
Curborough is possibly not one of the better sprint tracks but it would certainly give you an idea of what sprinting is all about and, as you say, it's not too harmful on the car because you are only doing a lap at a time.
As Dave has already pointed out they are totally the wrong size. If you are happy with Toyo's then why not stick with them, but check out the prices on Black circles and Camskill because they are a lot cheaper than you have indicated you pay for them.
On your estimation of 500 miles a week that's around 8000 miles you've got from them. This does seem quite low bit I guess it depends on the type of use you give them. Have you done any track days for example?
I can't confirm or refute that but one things for sure, it wasn't me. I was at Hethel before the M25 was completed and it used to take forever to get there when you were west of London.