Kevin

Tax is owed if you receive pay for work in the UK.

If the company pays you without an invoice, they will be unable to claim your fees as a deductable expense, which means they owe tax on top of what they pay you.

If you issue a contractor's invoice, you are liable for the tax.

If the boss is paying you personally, it is a "cash job" which means you are breaking the rules, just like so many tradespeople who offer cheaper "cash jobs" and avoid paying tax on the income. Usually this only appeals to "Joe Public" customers who needs work done on drains, roof etc for which there is no allowable deduction, so they might as well pay a cheaper price. While it is against the rules, in your situation you are unlikely to be "caught" unless somebody dobs you in.

Regards

JR