although it's not against the law, as you are 18, it is against his contract as a teacher..so basically he shouldn't be doing this, and if found out, will lose his job...
I think the best thing to do is wait until the summer, when I assume you will be finished with 6th form (it's only 3 months away), also this gives him time t ACTUALLY split up with his gf, if he's still with someone (even though it's not working out), you would be a fool to start going out with him, as you may end up his bit on the side...
Once he's single, and no longer teaching you, then a realtionship is fine, but until then, you BOTH are putting yourselves in a vulnerable position...
The dude is a 26 year old teacher, he should know this already. If he ignores all f the above (which im 99% positive he is aware of), I don't think that says much about his character tbh.