Hi buddy,
Sorry to hear about your health problems, hope you're managing well and you're on the way for a full recovery!
The most popular option under £1k for the Nismo 370z are the tail pipes:
Standard and louder - https://www.torqen.uk/nissan/370z/exhausts/exhaust-systems/1139-370z-aam-competition-s-line-axle-back-short-tail-exhaust-nismo-fitment-titanium--aam37e-tss-5t.html
Resonated / less loud - https://www.torqen.uk/nissan/370z/exhausts/exhaust-systems/9383-370z-aam-competition-resonated-s-line-short-tail-exhaust-aamc37e-tss-res-ss.html#/customize_your_item-nismo_fitment_titanium_5
Next popular option - full exhaust from Invidia Gemini - https://www.torqen.uk/nissan/370z/exhausts/exhaust-systems/1937-370z-invidia-gemini-cat-back-exhaust-system-hs09n7zgis.html
For fitting, any garage around your area should be able to fit the exhaust, we can ship directly to them.
If you're looking for mapping and a bit of more HP gain, best place to go would be Horsham Developments, see HERE. They can map your car with Ecu-Tek software, see HERE for details.
Any further question, feel free to ask here or drop me a message.
Adrian