If the certificate is recent, you can get a copy at the local registrars office - just ask them. If further back, you have to apply to a 'central office' - Garamond has supplied the link. You do have to pay, but it isn't too pricey.
Recorded is short for Recorded Delivery and it is just that: a record of the delivery. The letter isn't tracked and isn't treated as valuable. I don't believe it is even individually scanned when collected.
Royal Mail have this amazing arrangement whereby you can't claim for the postage when they've lost your stuff, only compensation for the loss of the article, and no consequential loss. All you can claim is the price of a replacement, up to a maximum of �28.