Try Haggerty Insurance. They are based in Silverstone next to the track and charged me 270 pounds inc breakdown recovery on my Tuscan 4000 miles per year. Sorry dont have their number to hand... google them.
Heritage - not had to claim yet (touch wood), but very competitive for me; no garage, no experience with powerful RWD sports car and no NCD to carry over.
A Mannings were the cheapest by a mile for me, depends on your history etc. I found 'normal' companies did not touch TVR's, or quoted stupid £.Good luck
I am with Classic Line, I got cheaper quotes from Hagerty and A Plan, called Classic Line and they beat them. Also had the misfortune to use Classic Line for a claim and they were very good, no problems whatsoever.
Another vote for classic line from me. I have also had good quotes from Heritage in the past, and also A plan. At the end of the day there is no substitute for getting as many quotes as you possibly can.