Bradley: For ease and variety you could do a lot worse than Aston Hill.
45 mins from Oxford via Thame and £7 to ride. Something there to suit all (the XC loop is a fun tech-fest and all the downhills are doable on a hardtail / 120mm rig, just not as fast as an 8″ beast) and with an OS map there’s miles of bridleways and – erm – should-really-be-bridleways to play on. Despite the body armour and full face lids, the regulars are a really friendly bunch too – don’t be put off by the larger than life characters.
For a big loop with loads of singletrack, chuck in Ashridge on the other side of Tring too.
I would offer to show you round, but I moved away a year ago…
Edit: go then when it’s dry though – it’s very interesting in the wet!