Well, you should be able to order a PDF of Frederick's birth cert, so they won't need to post you an actual certificate:
https://www.gro.gov.uk/gro/content/c...ates/login.asp
but they aren't doing the same for marriage certs yet.
The information for ordering a marriage cert says "Standard - £11.00, despatched on the 4th working day from receipt of order." or "Priority - £35.00, despatched next working day from receipt of order." (Though obviously there are serious delays at the moment, and the site says only to order if it is urgent.)