In order to teach private English lessons in Poland, which I'm planning to do for a year (I'm qualified!) is it legally necessary for me to register as a sole-trader, or can I just go ahead and start teaching and pay my tax back in the UK?
As it stands - you will be resident in Poland for more than 185 days in a calendar year, therefore you're liable for Polish taxation. There are murmurings that the impending placing of VAT on education is going to be met with a very tough crackdown on private teaching - so you would be best to register as a sole trader in Poland and pay the taxes.
It's an absolute doddle - just make sure you have someone Polish to guide you through the process and it couldn't be easier.
The crucial thing is that you need an accountant (usual cost - between 100zl and 250zl, but you should find someone to do it for 100zl+VAT) and you absolutely must pay ZUS and the tax office by the deadlines.
Usual practice is to start the business on the 1st of the month, then you pay tax by the 20th and the compulsory ZUS contribution of 350zl by the 10th.