Help - would like to rent a car for a UK tour

[ItaloBritt]

Hi I have a week off in early April and I was thinking of taking a nice driving tour of the UK - from Cornwall to the Highlands. Sadly I sold my 911S cabrio last year and I was thinking of renting a similar sports car - does not have to be Porsche as long as it is a convertible. Any suggestions? I looked at AVIS prestige but they have a limit of 100 miles per day, so useless. Thank you!