You need to open another bank account with a different bank - preferably one with no overdraft. Then use that for all your wages going in, and make arrangements with HSBC to pay off your overdraft. Even if you can only afford £1 a month, at least you'll be doing something.
If you don't try to pay anything, HSBC will start charging interest, and then may add charges to your account (you might have heard about reclaiming bank charges, but they'll still apply them in the first place). Then they will send you increasingly nasty letters. You'll probably end up with a default notice which messes up your credit rating a bit. After a while, they'll go to a debt collection agency, usually Metropolitan, who will employ their own nasty tactics to get you to pay. If you still didn't try to pay it back, they would eventually go to court and get a CCJ against you.
Bailiffs cannot come round until after you have a CCJ. They may try to scare you by talking about sending debt collection agents or similarly worded things round, but these aren't bailiffs. They have no more rights than the postman.