Lots of different questions here.
I declare my earnings and pay tax. It's fairly simple to do, you just need to go on the HMRC website and follow instructions. Keep around 25% of your earnings aside to pay your year-end tax bill.
If you make under the threshold you'll be able to claim tax credits. But with all the fuckery the tories have done I have no idea what that threshold now is. (sorry for politics not sorry)
As to how you declare yourself, it's varied. If at some point you may need to find a private tenancy, then it's best to stick to massage, etc. If you only care about paying correct tax, then be honest, because you'll be able to claim ALL your expenses e.g. condoms.
Have you dumped your boyfriend off? He doesn't sound very helpful. Are you safe?