All death certificates are in the public domain but no online service has any better access to them than the general public do. So, if you use an online service to obtain one, you'll end up paying extra for something that you could have done yourself directly anyway.
There's no way of doing it for free though. The charge for issuing a copy of a death certificate is £11, plus £3 for searching for it. (You can avoid the £3 search fee if you've got the 'GRO index reference number' but the only free online search facility to get that number currently doesn't go past 1992):
https://www.gov.uk/order-copy-birth-death-marriage-certificate
(That link applies to deaths registered in England and Wales. Click the links from that page to see how things work in Scotland or Northern Ireland).