Locksmiths charge some fairly hefty figures for emergency call-out work so, theoretically, you could make a good income from specialising in this line of work.
You'd probably need to have some capital behind you, initially, because there would almost certainly be a period before you started getting much work.
Despite the presence of websites advertising such services, it's almost certain that the vast majority of people seeking a locksmith will turn to Yellow Pages. So you'll need to invest a reasonable sum in placing an advertisement with them and then wait for up to a year for the next edition to be published. Until sch time as you've got an ad in Yellow Pages, I doubt that you'd get very much work (unless, of course, you either open a shop or persuade someone to use your services on a sub-contract basis).
The Learn Direct site usually provides a reasonable guide to potential income. Their page on becoming a locksmith has this quote:
"Trainees can expect to earn around �10,000 rising to over �14,000 with experience and training.
A self-employed locksmith can earn in excess of �20,000 a year."
You might find other useful information on that site:
http://www.learndirect-advice.co.uk/helpwithyo urcareer/jobprofiles/profiles/profile414/
Hoping this helps,
Chris