Watching any live TV in the UK, even when avoiding all BBC channels, is subject to this mandatory payment. Hence it is, by any reasonable definition, a tax.
The real difference between VAT and the TV licence is that VAT goes to the government via the Inland Revenue (?), while the TV licence goes to the BBC via Capita. Because it doesn't go to the government it's not a tax. I've already said it's tax-like.
Call it tax-like, but it is not a tax, and mistaken terminology is unhelpful when campaigning to change the licence fee.