Do you know where you will be living yet? It might help to find other riders local to you.
Riding in North east london – Epping forest. London clay basin, wet, flat but can be pleasant xc stuff, you have to make the most of what htere is if you want 'technical'….you become expert at stream/ditch crossing and getting over fallen tree trunks. I ride there occasionally (parents live in Epping so I tie in with a visit) and can show you around, or Heales cycles do a regular ride. Train from Liverpool street goes to Highams park and Chingford, both in the forest.
Swinley to the West – flat, xc but more technical than Epping and better draned I believe
North and South Downs, train from Victoria to Dorking for the North Downs (Leith, Holmsbury, Pitch hill) XC riding but quite a lot of technical bits about, jumps,drops etc. I normally only bother with the hardtail but have once taken the bouncer and there's enough to give it a bit of a thrashing. Nirvana cycles is Westcott do regular rides.
There's also Jedi's Herts shore.