Indeed, if my cream work was all close together over a three-day period (for instance), I could do £400 a day for those three days.
However, it isn't. It's interspersed with other work and spread throughout the round.
I see no reason why, in time, someone wouldn't be able to have most of their work at those prices if they were prepared to work hard at refining their round.
The problem is the old one about a bird in hand worth two in the bush though. Would I be prepared to take on one larger, very-well paid house and drop maybe three or four moderately-priced semis to make room for it - especially when there is a greater chance of losing the well-priced job if someone puts in a far cheaper quotation. Generally, I wouldn't. I keep both lots and work even harder to make time for both. I imagine that most shiners would.
If willing to take on staff, it's not an issue, but if wanting to remain a sole trader, there comes a point when you can't physically do more.