The problem is that “decently light, durable, not too niche” 140mm full suspension bikes don’t cost £450 any more sadly.
For that money, decently light will mean older and more used, with suspension that may well be harder to get serviced these days.
Durable would mean higher quality and less used, which blows the budget. Go lower quality and it will mean less durable and heavier.
Not too niche – I would argue that QR forks with straight steerers (which is what you’ll find on older bikes) is now pretty niche.
For me, the sweet spot for used full suspension bikes would be something in the 8/900-1200 range, for a bike that will be decently nice and enjoyable to ride.