Where to find 50+ year old people for guerrilla usability testing?

Challenge:

  1. mobile application targeted to people who are 50+ years old
  2. we would like to get as much feedback as possible, that's why guerrilla style testing would be best

We have prepared a prototype in English and therefore we would need to test the application in a country, where English is a first language. Our company is based in Europe, so we are looking at UK and Ireland. We think that we will make a 2 days' trip. Big question: how to find the right target people and how to "recruit" them on the fly?

We are thinking about some places for senior centers or places where older people spend time (cafes, libraries, etc.).

Do you have any suggestions / ideas how to plan testing like this?