Great question????
3 points come to mind, none of which specifically is my reasoning, more a combination of them…
If someone came up to you on the street and asked to take your photograph, would you say yes? Would you question their motives?
(OK, it depends, so maybe they are curious, they ask nicely, you get a good feel about them. you say yes. They are gruff and rude, you say no).
What are they going to do with that photograph?
Something about being British and a cultural hang-up about personal space?
After a while it gets really tiresome having people ask / photograph you. I’m a tourist, not the attraction 

