Commiserations to your friend sittingbull, that must have been nasty and really scary.
Bite back spray isn't any good because as you've already discovered, it's not readily available. You can buy them individually from police-supplies.co.uk, but it states "Conditions of sale restricted to Police, Emergency Services, County Councils, Animal Welfare Officers, the Royal Mail, Utility Workers, Veterinary Practitioners and Bailiffs"
Best thing to do is stop and put the bike between you and the dog, stand tall and bellow at it to stop. If it's a real concern for you, try as stewie suggests, keep an extra water bottle handy and add some washing up liquid to it, then squirt that at them.
I like dogs, but am wary of ones like staffies because of the damage they can do if they do bite you. I know a couple of people who have had to get their dogs PTS after being attacked, one by a staffie, the other by a bull terrier. Incidentally, although I've worked as a both a postie and a meter reader, I've never once been bitten or attacked by a dog. A cat once caught me a cracker as I was putting mail through a door, cats - the silent assassin!