I usually work on 20km/h including breaks for audax, or other long rides where I am not taking much luggage. Its 240km from my house to Cardiff, which would also give 12 hours. Probably similar for 'Teef and Uxbridge / Hillingdon / West Drayton is about 10km or half an hour down the road.
The Severn Bridge is then about 180km from there, or about 9 hours. So if we were to meet in that area at 9am, we could expect to join User482 at the Bridge around 6pm, to get to Cardiff for 8:30-9pm. That would leave loads of time for an evening meal and battery recharge before the main event.
However, if either of you have a particular preference I wouldn't mind setting off a bit earlier or even slightly later.
'Teef, where does it make sense for us to meet? I was thinking of exiting London via Uxbridge Road as far as Hillingdon, then crossing the M25 at Iver and then skimming Slough to Datchet and Windsor.
Edit - what might work better, esp I think for you, Adam, is a more northerly route (which 'Teef has been keeping up his sleeve) so we could meet at Harefield or Chalfont St P (at say 9am) to head through the Chilterns to Beaconsfield and High Wycombe, Christmas Common, etc.