It will be used mostly for XC & a few bike parks/trail centers (Antur Stiniog/Black Mountain/Llandegla/Betws y Coed etc)
£500-£600 should get you a pretty good used bike but you really need to decide on a main use.
When you say used for XC what level of racing do you anticipate? (or do you mean riding XC type trails?)
An out and out XC bike might be a bit of a job to handle on trail centres and unless your enjoyment comes from blasting round against a stop watch or other competitors likely less enjoyable. On the other hand it will eat easy miles and fire-trails etc….
There is a big difference between a XC race machine and the new generation trail hard-tails. Probably a bigger difference than between a trail HT and short travel Trail FS.
I personally find that I have more fun on a trail specific geometry unless I’m basically riding fire trails… It’s not as fast on a stop watch but I have a bigger grin.
If you get a bigger grin from a stopwatch then obviously the more XC geometry.