As an keen but casual touring cyclist I'm interested in getting a little more competitive and also keeping myself comfortable over longer distances. So I'm considering getting professionally fitted, as from others' experiences it sounds good. What should I look for in a fitter and where in the UK (preferably the South) can I find a good one? I'm wary of going somewhere that will be mostly focused on selling me new parts or a new bike.
Also, would the measurements be in any way transferable to another bike? I ride a Marin San Alselmo and a Thorn Tandem, plus I'm considering something properly race-roadie as my N+1.
EDIT: I know there are a few sub-questions here and many people won't be able to answer them all, but I'd also be appreciative of partial answers.