As a former teacher, I know that most of the big unions would suggest that you should charge at least �30 per hour for your services. I also know, however, that there's no point pricing your services out of the range of those people who might want to engage them. A quick web search has shown me that driving instructors (who have far greater overheads than you will) seem to be charging around �20 per hour at present, so that might be a more realistic figure to charge.
The tax man will want 22% of everything you earn (because your tax-free allowance will have been used up through your college employment). There is almost certainly some information about this here:
http://www.hmrc.gov.uk but the HMRC website is notoriously difficult to navigate. It would be far simpler to speak to your local tax office. (Basically, this just means that you'll have to fill in a basic tax return every year and pay the bill when it arrives).
There should be no need to tell your college about any outside employment unless it conflicts in some way with the interests of the college. (e.g.. it would normally be regarded as unethical to provide private tuition to one of your own students. It might be wisest, however, to mention any private tuition offered to anyone who attends the college, even if they're studying a completely different course to the one which you teach. In particular, the college might deem it inadvisable for you to place yourself in a situation where you'd be alone with one of the college students, of either sex, irrespective of the fact that you don't teach them at college).
Chris