Any question answered...

Just go for the cheapest hire car company, they are all much of a muchness.
Bear in mind that it's the London to Brighton this weekend which will mean rolling road closures in Sussex, which might affect you.

Forgot about that. The majority of the 24 course is either side of the L2B route so it should be ok. I might change my plan to Fri night instead though to avoid the gettting out of London bit.