I think the main issue is claiming benefits and working, because you'll never know how much you're going to make, though I don't know anything about tax credits.
I declare absolutely everything, both in terms of income and outgoings that are claimable expenses, i.e. a portion of utility bills etc., condoms, makeup, stockings, mobile and so on. You'd be able to claim any rent you paid your boyfriend if it was purely for work purposes.
I'm not going to ask how you have £3k+ in monthly expenses, but is there any way you could bring that down??
You do have a few months' grace before you need to declare yourself as self-employed, so you could always try it for three months and see how it goes?