Lala, one of your (many) problems will be that if somebody drops you in it, just saying that it's OK because you haven't earned very much without any way of proving it isn't going to cut it with HMRC; they won't believe you, will calculate what they think you could have earned based on your hourly rate (and yes, your age, appearance and so on) and it's up to you to prove them wrong which you won't be able to do without keeping proper accounts and records. When they decide you could have earned ?100K (bearing in mind they have no idea how our business works), failed to declare it and bill/fine you accordingly, what will you do then?
Income tax isn't like VAT - you are obliged to register whether you've earned X amount or not. I'm sorry if I don't sound very sympathetic, but I left home at seventeen having already been in full time factory work for a year and as far as I'm concerned if you're old enough to work as a prostitute, you're old enough to run your business properly and without sponging off those less fortunate.