What you saw wasn't dynamic pricing. The fact that the currency changed almost certainly means that the point-of-sale changed - and airlines have been doing different pricing for different PoS basically forever.

Why did the PoS change? That might have been due to the login, or it might have just been random website quirkyness (more than once I've been quoted priced in HKG, only to research and have it jump back to USD - and others here have seen the same).

Presuming you are actually in the UK, it's likely that even if you had tried to purchase the US$ price that it would have failed based on you using a UK credit card. The fare rules for that fare probably say that it's for sale in the US only.