I live within commuting distance of London UK, tho I don't go there very often.
Where I get streetpasses (currently): - my local town shopping Mall advertises free wifi - and it acts as a relay point for Nintendo so I can pick up 10 there (not many if any are now playing acnl tho).
Most Game shops also have a wifi connection and act as a relay point for streetpasses.
And that is about it. When the 3ds first came out in 2011 there was a lot more activity.
Currently I get hardly any acnl streetpasses, when the game first came out in 2013, I could get as many as 16 (acnl will hold up to 20 before you have to clear) - 6 show in HH plaza and the remainder will be by their houses.
I was in London a few months ago, walking around, incl areas like Picadilly (lots of tourists) and still did not get any acnl streetpasses. Got the 10 in Mii plaza - but I had a feeling they were from a Starbucks stop off! (ie it had free wifi & acted as a relay point).
Airports can surprisingly be quite barren for streetpassing.
Conventions are brilliant - comic con - whatever.
Dundee may host streetpass events - my local town does, its small not a city, yet still holds them.
http://streetpassuk.co.uk/upcoming-events/
I'm an adult, but even if I was at school, the schools around my area all ban handhelds being taken to school.
Dundee is a University town - there is nothing to stop you going around the campus area in the hope of catching a few.
I presume you would not get into buildings (eg students union), you don't need to, just being around outside would work.
Dundee's streetpass community seems to be university based -
http://streetpassuk.co.uk/community/streetpass-dundee/