OP I'm really concerned about his comment that his business situation is "none of your business". £280K debt or no, I couldn't marry anyone who wasn't able and willing to be completely honest with me about his financial and business circumstances.
I'm not suggesting that you should be delving into the business accounts in detail, but if you are going to be a marriage team together, at least a vague idea of what is going on is, to my mind, essential.
I am a lawyer and I regularly advise company directors on giving personal guarantees. I also act for banks recovering those sums. If he'd given a personal guarantee and the Bank was actually calling on it, they would have done so by now. So it doesn't sound like a personal guarantee debt to me.
If he's set up a business with his friends and he's know for being a bit flash with his cash, do you think they have brought him on board to fund their hopeless business idea?
When you marry, your assets will remain yours unless you want to merge them or if they passed under a will. In the event of a divorce you would probably hang on to them, but a pre-nup (although it is only persuasive, not binding) would help.
I'm also concerned about you saying that you want to build up a good credit rating. If your finances become linked (because you share an address or you have a joint account for the bills) your credit rating may suffer because of him. There is some good stuff on the Money Saving Expert website about this.
Please think very carefully before you agree to marry him.