approaching strangers to take a shot there and then. What shall I say?
Just ask them nicely. You can say that they compliment the background nicely and you’d like to photograph them or explain why you are taking the picture, or even say that you are doing a study of people/cultures in Exeter and would they mind if you took a portrait for your study. People under 18, you may want to be careful approaching young girls but if they are with an adult talk to both of them and explain. Same with children (like real young) ask their parents for permission. Make sure they know that if they say no it is completely ok.