Have you tried Adrian Flux? they are very good for me, and there is no problem with modifications.. I was insured with Churchill and they didn't even let me put BMW alloys on as they were not part of the factory spec for the car.
My first insurance company was Eagle Star, they had a fairly good rate for my first year, but that was with a Honda Civic 1.4i.
Here is a list of other insurers in the UK >
http://www.talkaudio.co.uk/vbb/showthread.php?t=90347