Decat then remap is a good upgrade and done right gives a nice safe boost in power and responce, tbh going from stock to decat and remapped gives the most noticable difference too how they drive imo.
Beware where you go and who you get to map it (avoid dungkhan greaham aka racedungnamics), also advised to go somewhere that does the mechanical work and remap to prevent any sort of blame issues if somethings not right. Beware of people promising 300+ mega power increase etc, the stock turbo will run out of puff about 260-280bhp ish. Results vary from car to car so you could find you end up at 270bhp or 260bhp, the biggest difference you will notice though is the responce which cahnges the driving experiance totaly. Also it will take a few hours to map it as they need to be custom to your car, not just a flashed on generic map. If its mapped on a dyno, it needs to be road tested after as dyno conditions and road conditions, while simmilar, are not exactly the same.
You'll prob find you need a fuel pump as well and prob worth doing the airfilter while at it as they arn't big money and give a little (very little, 1-2bhp if that despite whats claimed) and they do help with airflow.
I'd expect a full exhaust, map, fuel pump to be £1500-2000
for example https://scoobyclinic.com/performance-tuning/