london based - go for RedDot. they are in enfield these days. they know their stuff and will pretty much tune it to your spec. costs about £300 for full remap with before and after print outs I think.
don't go for one of the ****ty places that say they come to you, you want your car put up on a proper dyno and done properly, not driven down the road and hope for the best type job.
fuel consumption should go up as well as power although maybe smoke a bit more, although mine does smoke like hell already.
MOT implications are nil, its fine. insurance it should technically be declared, although I bet most people don't. maps are pretty much untraceable and I've never know an insurance assessor check for one, however it would invalidate your insurance if discovered. also note that if you are insured isurers are legally obliged to pay out on the third party liability, it would only be you thats not covered.
I do know Pete, the mad austrian with reddot did managed to tune a GT 1.9 jtd-m to 250bhp and **** load of tourque with other supporting mods.
depends on what other mods you've had done I think the 1.9s are good around around 185-200bhp