I am based in the UK and am a sole trader.

Is it lawful for me to pay business money into a personal account? The account is not limited.

So long as you are not trading insolvent and not attempting to falsify your accounts and cheat the tax man what you do with your own money is your affair, however it is unwise to mix personal and business funds and if you have a company structure it would be illegal. If you want detailed legal analysis of your situation consult legal counsel, remember the advice you get for free is worth what you paid for it