Yes of course you have to pay it back, why wouldn't you?! They should give you the same terms as normal, so once you are earning over £21,000 a year they will automatically write to you and give you the payment plan based on your earnings. You start getting interest charged on the loan from the very first year you take it out - it's supposed to be based on inflation - so whether or not you are in a payment plan they will send you an annual statement saying how much your account stands at. If you have been given any maintenance grants too, eg if your family has very low earnings, then you should check if that comes with the condition of you finishing the course. Usually they just say you have to finish the year (whether you fail or not, as long as you attend, finish and do all the exams etc) but if not then they may ask for that back right away, or at least ask you to begin a payment plan on it immediately, regardless of how much your earnings are.
If you chose to go back to uni either by resitting the year or by going on a different course you still could, as they allocate 4 years worth of loan to each student, so it leaves your options open for the future.