I don't know England geography, but I know many many members of the UK DeTomaso Driver's Club and they, without exception, speak very highly of Johnny and Roger. Offhand I can't think of any other shop that really knows DeTomasos.
There is a fair number of DeTomasos in England, and in the past the club has been quite active, with monthly events. I'd ring Roger, who is the club secretary, to see what is scheduled between now and the end of the year. He could also probably tell you who lives in your neighborhood.
If you can make it to the Goodwood Revival in 3 weeks you will probably be able to meet everyone in person, as it is highly attended by DeTomaso People. It's a highly worthwhile event regardless...
Welcome to the club!!