Any warranty would require
you to deal with the company providing the warranty, either the manufacturer or the insurer if it's a third party warranty.
When dealing with the retailer you're operating under your legal rights under the consumer rights act and whatever published policy the store themselves have:
http://www.which.co.uk/consumer-righ...mer-rights-act
Basically they can choose to repair or replace the board whichever best suits them. But it has to be completed in a reasonable time, without significant inconvenience to you and you're only required to allow them one attempt at it, after that you can request a refund (possibly a partial one if it's more than six months old).
Of course you can request other resolutions if they're better for you but the store isn't required to oblige you.