I used to file VAT quarterly through the HMRC website when I had a small consulting business in the U.K. (back when that was still in the EU). It really wasn’t very complex.

I did use an accountant for filing my company’s annual accounts and taxes, though when I was a sole trader I just used the HMRC tax website to file. Far simpler than any of the options I’ve tried in the US, even as a regular employee (which you usually don’t have to file anything for in the U.K.)