I've sometimes found that local register offices offer a more personalised service than the GRO - one that I've dealt with was happy to discuss on the phone details such as an occupation or a relative's name to check they were issuing the right certificate.

However, you need to bear in mind that with a marriage they'd only be able to help if you can first establish where it took place, as they can't do a long search in all the registers from every place of worship. The indexes at UKBMD can help in this, but even if the office/county isn't included there, most offices are happy to explain on the phone what they can and can't do for you. Sometimes there might be local consolidated indexes that aren't online for public viewing.

You'd also need to bear in mind the extent to which local offices are open and offering their usual services at present, but no doubt they would explain this too.