ba.com
bmibaby.com
easyJet.com
flybe.com
flybmi.com
jet2.com
thomascook.com
virginatlantic.com
All these UK airlines seem quite happy to use '.com'
.com is the internet domain normally associated with the USA and .co.uk with the UK, which is the most likely the reason.
As you stated
.co.uk is for UK based companies/domains,
.com however is a worldwide international domain.