Quite simply, what affects point of sale?

I'm UK based, with UK Exec Club and UK credit card.

If I book a flight starting in the USA (e.g. USA - UK / UK - USA) is that UK or USA POS?

Similarly, for an ex-EU, would the POS always be UK for me?

Finally, if I phone another country's sales line (e.g. USA) would I still be purchasing as UK POS?

All questions could be slightly irrelevant as all flights I've searched have the same availability across all fare buckets for different POS.