The CAA also regulates UK airspace and everthing that happens in it. If the CAA has a more stringent requirement than the FAA then the US carrier must meet that requirement.

As an example to fly in IFR a CAA (EASA now) certified aeroplane needs an oldie worldie navigation aid called an ADF (basically an AM radio!). The FAA haven't required this for years. In order to operate in UK airspace the US carriers initially needed to have the ADF receiver installed to meet UK regulations. In this case there is now a formal exception to the UK rule so that US carriers don't have to meet that requirement. However, it's for the CAA to make the exception not for the US carriers not to comply.

The feature of aviation is regulation is different from state to state - all regulation applies to an aircraft/crew - in the case of a contradiction - the worst case applies.

The CAA has a "policy" on electronic devices - It's policy isn't within the realms of the Air Navigation Order - which means it's not law.