The challenge for a GP surgery doing these is that only the GPs and some relatively limited other members of staff are fee earning whilst facility and support staff costs are high. The total costs to provide a service ten years ago were estimated to run around a £250 an hour break even point per GP, so doing ten minutes of GP work for £35, plus whatever admin staff time is used on the job is never going to make any kind of business sense unless there is unused capacity being paid for that can be monetised.
If you have a set up where all a doctor does all day is certificates from a home office without the same overheads a much lower charge out rate becomes viable, so long as they can get suitably efficient access to records.
I had a long chat in the pub with a few mates about this when the system was brought in as we thought it could be a good sideline to set up, but back then we couldn't find a way to make it viable at a competitive price. BASC then set up their own system which again when we looked at it would run at a loss unless it was using time and resources that would otherwise have been paid for without raising any income.