I added both Nationwide VISA debit cards to my Passbook this morning without issue, but have yet to actually pay for something.
I'm thinking about just going to McDonalds up the road to try it out
I did pay for something on Sunday with my Apple Watch though, but it wasn't using Apple Pay, it was using my Greggs card. There was a free donut offer on, so I showed the guy behind the counter the offer and asked which donut it was. He said any of them and then asked if I had a barcode, to which I presented my Apple Watch to the reader. A split second later there was a bleep and it was all done. He grinned and said 'That's so cool!'. I walked out feeling pleased with myself!!!