From a quick Google search, IVIG infusions are a treatment used in the NHS, so yes if a consultant started you on that treatment you would have it for life if necessary and it would cost you nothing.

If you live somewhere with consumer rights, by law you can return it no questions asked within 14 days of receipt (actual length of time may vary, UK is 14 days)

I’d never signed a credit or debit card in my life until I travelled to the US last year, and then I only did it because I’d heard stories of places confiscating cards if they weren’t signed. Don’t know how true they are now but I wasn’t risking it.