Not sure this is an 'old British saw blade', looks to be a castoff Starrett "bi-metal cutting" blade from an electric reciprocating saw. If 5-10 years is to be considered old then this qualifies. They generally come in packages of 5 at the local Home Depot if you care to look there. Milwaukee makes particularly good ones but none of them are thick enough except maybe for a modest filet knife!
It is a Starret blade, says 'Great Britain' on it too and has been stored away for years. It's about 2mm thick though so it'll be good enough for a knife blade. Also, home depot is american, and not all of us are american.
