This link will help you. https://www.gro.gov.uk/gro/content/certificates/most_customers_want_to_know.asp#FamilyH...

I could be wrong as have no actual experience, but I think I'm correct in saying this is still the least expensive method of buying them, or at least best "value", they've increased subtantially in recent years.
http://www.genealogy-guide.org.uk/marriages.html

I think you can pay others to do it for you but just get same service but at extra cost of course., heres a randomly selected example of what's on offer
http://www.ukcertificates.com/marriage-certificate/

I welcome anyone else's views/opinions based on actual experience as to the best method, please ?