A word of advice if a tuner tries to hard sell you an aftermarket remap (ie. superchips, bluefin) to get to 220bhp, walkaway. Remaps are for power delivery and smoothness not to achieve your high powered goals. You only really need one if you get a bigger head.
A 15% pulley and colder plugs was fitted to a guys cooper s by 1320autos (think it was ewmarine or tristo) and his put 216bhp down on the rollers, i did the same (pulley and plugs) before I came across 1320 and it cost under £300!!! Awesome first upgrade.
After that it's best to let it breathe better and give it more fuel so an uprated intercooler (GRS approx. £400 or if your on a buget get a one off Cooper S GP if you can (I paid £200 for mine)) to help it cool down and breathe better. Then for more fuel bigger injectors (Works ones are 380cc and are what I use, standard are 330cc).
Your probably looking at £1500 tops for the lot fitted (you can do the intercooler yourself easily) and around 225-230ish bhp
and some extra pennies saved to get a nice sounding exhaust/intake system
Paul at 1320 autos is one of the only tuners that will post up the basics you need to make power and all the add on rubbish you don't need to buy. Because this gives us mini owners more of an insight into this other tuners aren't happy because it leads to them getting more questions and less money out of us. How many garages do you know whos tag line is "It's not how much you spend, it's how well your spend it".
The tuners are banned for a few weeks for arguing, 1320 get a lot of stick from other tuners because the give out more info about what we need to spend than some tuners would like.
If you want to spend a lot of money, get good service and decent power go GTT / Lohen, if you want to spend less money, get good service, make the same power (or maybe more) and learn a hell of lot about your car go 1320autos.
Good luck.
PS. It's also more about location but most sponsors on here will be happy to answer your questions even if your not getting things fitted by them.