My advice is go to a reputable place (that's recommended by folk on here) that is local(ish) to you
You can pay between £250 - £600 for any of the reputable maps on this forum. If you like brands, pay £600 and get a Revo sticker for your window, a little less gets you super chips, and for around £250-300 you can have a PDT or R-Tech map (the latter I very much recommend).
At the end of the day, all maps (providing set up right) make about the same power (dependant on mods). For example, my £300 R-Tech custom map made the same power on the dyno as another GTI with a Revo map. . .
You pays your money, you take your choice