The £900 price you quoted is usually about the full fitted cost from Volvo for a detachable towbar for xc70 or v60 which aren't that different, really.
It does look as though there is a shortage of official detachable ones at the moment, though. As long as you get a towbar of reputable make you should be fine as fitting is very straightforward. E.g.Thule/brink(original manufacturer for volvo towbars), witter, etc. Check out
https://www.pfjones.co.uk/tow-bars/v...7-towbars.html
They even have your oem electric folder available.
Highly recommend using genuine volvo parts for the wiring harness and towing module although, depending where you get it done, it could cost anything between £40 and £100 for the required software.
Check under xc60 and xc70 and you will find loads of info regarding installation companies and pros and cons of non original wiring.