It's always a tricky one, - we've got a tiered scheme, so if a school uses us on an 'ad-hoc' basis, they pay slightly more, but if they've got a contract, then the hourly rate is lowered. There is definitely a world of difference between a self-employed person providing the service, and a small business, just a couple of the additional costs that spring to mind are:
1) Business premises - we've got an installations office with a workshop and garage, and a support office
2) Staffing - we need to bring in enough to cover the admin staff salaries, and to cope with the inevitable 'slack periods' when the work drops off
Plus there's the fact that with a single person providing the support/consultancy, if they are ill, or take a holiday, then it's often tricky to get cover (I know some people manage to do it though).
I tend to view the role of external support/consultancy as supplemental to that provided in-house by the schools, and in fact we often get called in to train up school techs on particular areas (such as Exchange, or Group Policy stuff).
I was self employed consultant in 2002 and charged £35 an hour, which worked well for me and clients although I suffered the same fate as anyone who did a good job, as mentioned here before, it would be rare to see the same client more than 2-3 times in a year. In the end I had to find a contracted full time job somewhere that guaranteed a monthly income. Enter school support job.
If I hadn't ditched it when I did, I wouldn't have a pension pot now and I wouldn't have been able to meet rent as work could be very busy then months of barely a peep out of anyone.
Consultants who are worth their price are few and imposters are many. It's often hard to tell if a consultant is any good until after they've finished.
With a bit of initiative, me and my team of 1 get just about everything done ourselves. We stopped our support contract as they were no good to us, they only provided the kind of service we could provide ourselves and no longer covered replacment on any kit. We spent last year's money replacing kit and haven't looked back. Of course, having a decent budget for rolling upgrades makes that kind of strategy easier.
£15 an hour is a good rate for support, consultancy is a different game. Like it's been said, after covering costs if you can afford to charge £15 an hour, you're doing something right, but may be working a lot of hours before you're a rich man.
All the best