"..it's hard to pin down the exact reasons and price differences"

Generally it's down to VAT @ 20% and import duties, with some exchange rate padding.

Example:

Apple iPhone SE is £359 in the UK and $399 in the US.

As of 15:00 on 06/09, the $399 is ~£298. Add 20% VAT and you have ~£358.

EDIT: Exchange rates play a big part.