I've had my coop for just over 2.5 years but am now looking to take the power up to sensible levels, say 250 bhp.

Just about to fit a bailey double piston trumpet dump valve
and has a K & N filter. FSH and has mainly done motor way miles racking up the mileage, no smoke from turbo or any problems.

I am looking for a place who can do everything and just drive away.

To include say:

Boost gauge (not screwed to dash)
Gtec

and any other mods that would be suitable to keep it running safe.

Any suggestions on who can do this and properly?

if you look here

You'll get a list of the good garages that people use on here, find one thats close to you and talk to them.

Or alternatively the bits you are asking for are not overly technical. If you don't want to try it yourself, maybe see if there is another forumer who would be willing to do it for you for a few quid

As Jim says, they are both jobs you can do yourself easily, there's probably even a how-to in the FAQ section on here