Tax laws uk

You don't pay tax per month here. The tax year ends on the 6th of April and you have until 31st January next year to file and pay tax) and until October to register).

If you're merely trading you'll pay capital gains on:
Withdrawals + final balance - (deposits + starting balance).

You have a personal limit of £1000 (I think) anyway that you can earn before having to file anything.

If you have an income you'll probably already have your utrn (unique taxpayer reference number).
You just need to apply to be able to log in to the government gateways website and then register through that for self assessment (you can search for those on Google or gov UK).

If you file or pay late it doesn't really matter too much but try to file on time regardless - they give quite a lenient amount (8 months).