Elizabeth II On Horseback Silver Coin 1977. (dg Reg Fd). It Has No Other Markings As To What It Represents. Is It From Canada? What Is It Worth?

I recently found one of these coins whilst at work ( I'm a decorator) According to other information I could find on the internet I believe it is either worth its face value £0.25 or £5 because of some change at the bank of england. I'm keeping mine in hope it may be worth more for my children to sell :)