For subscription sites I look for an alternative. I've found FamilySearch immensely helpful here. For certificates from the GRO I quite often use the GRO reference i.e. Country, District, Year, Quarter, Volume & Page and then reference a picture of the index available through FreeBMD. For Australian certs I use the index references that are available on the State run BMD sites.
When you viewed the original did you transcribe it? You can reference your own transcription as a privately held source along with the citation of the index to point people to where the original can be found.
Does this help any? The less you can reference a subscription site the better although sometimes that's all you can do.