In reply to Tyler:
Surely that's a statement of the obvious? Probably around 30-35 hours a week of training (plus time on rock at the weekend) is necessary to truly reach maximum potential and you can't fit that around a full time job.
However, it's more complicated than that. Some people would never be able to withstand that intensity of training, and 3-4 hours a day, 2-3 days a week might represent the best amount of training to reach their potential, and that's easily done with a full time job, given sufficient commitment!
To be honest, for most british climbers it's not the amount of time they spend training, so much as what they do in training that limits their ability to climb routes. I'm talking about a lack of structure, ignoring base endurance work, too much resting in a session, etc, etc. If you're a full time climber this doesn't matter so much, but by getting this right I think it's quite feasible for the talented to climb 8c+ with a full time job.