Space must be a massive factor in Londinium.
Imagine asking The Enemy (one's Mrs):
"Hi love, mind if I spend the next three weeks pulling this old bike apart and putting it back together again in the front room?"
For some, it is worth the extra Oxford scholars to have someone else do it.
I also think that these places do not just knock out complete bikes, but hard to source parts and advice for people that have had a go and are stuck.