if I was selected for direct entry after 6th form, how long would it be before it gets serious and I'd have to start proper training?
If you've been accepted to go Direct Entrant as soon as you've finished your A2s, then you could be going to IOT the second they're over.
In theory, it is possible to finish the exams on, say, Friday June 12th and be starting IOT on Sunday June 14th. In reality, I doubt that would be the case - they want to know you've definitely got the grades/entry requirements first, too. But anything more than 5-6 months is unlikely.