I would always tip what I was comfortable with. Having used charter boats a fair bit here and a couple of time overseas - I go with what I am comfortable with and, more importantly, what I can afford.
After all, imagine if this is your " once in a lifetime trip" and you can just afford the guide in the first place; £10 may be truly all you can afford.
A good guide would come across as very grateful for anything from a pint to a pound and upwards.
Afterall, a tip should not be a given or expected - you charge a price and THAT is your price. A gratuity is a nice wee add on.
Why is it that Guides get a tip and an Airhostess does not?
Or your GP or a brain surgeon?
Why is it a waitress expects a tip or a bell hop but not a shop assistant or the Chef who cooked you meal?
Interesting question and a can of worms perhaps but, all in all, I would say to tip whatever you want to or can afford.
Or just go with the standard 10-15%