No, by all means there's Jabba, APS, Superchips and a few other choice genuine custom tuning houses. Very few people have proper ability and more importantly the R&D, those that do are well known and you can expect quality, research and you will find. Lots of people claim to have this level of ability, most just talk in jargon to fake it.
Then, there's the so called "custom" tuners who buy generic crap off the net which only changes a few parameters on the car to fudge an increase in power. Usually accompanied by a dodgy dyno/claimed output, these cost less, but produce less and will (unless I'm proven wrong) be destructive in the end.
Be wary of anyone doing fly-by-night sh*te, you've got a £xx,xxx car, scrimping on a couple of hundred pounds which represents a minute percentage of your car's value will NOT pay off in the long run. Buy sh*t, buy twice.
Stokey, did I forget them?? Oh well....