I had the English to Welsh conversation switch happen to me too back in the seventies in Llangefni, Anglesey. I have never considered it to be representative of the Welsh a whole though, I have several Welsh friends.
If it wasn't for the fact that so many people from other countries speak English I'm sure it would happen in the rest of the UK too. As it is people just wait until the 'foreigner' has left before moaning.
My brother-in-law and his then girlfriend were once on a train in the Czech Republic where two middle aged women in the same compartment launched into an offensive and critical tirade against the dress and behaviour of foreign tourists. Upon reaching their stop, my brother-in-law and his girlfriend briefly spoke to each other in Czech and left (he was there teaching English in a Czech school and she was Czech).
Human nature the world over. The same is true of tolerance and friendliness too though.