I have seen this before - if I log on to united.com it knows I'm from the UK (UK billing address, MP address, etc.) and gives me prices in GBP. However, if I don't log on to my own account but still use the site from the UK, sometimes I get prices in USD.

So, for all intents and purposes, quoted prices can be different depending on whether you are logged in or not. Whether one can actually purchase the cheaper USD price with a UK billing address is another matter.

The .co.uk website is long since defunct - just redirects to the US version of the united.com site. But the top right hand corner of the united.com page will tell you whether you are on the UK or US version of the site. If you are not logged in you may have to manually select the UK version of the site if you want GBP prices, even if using the site from the UK.