I live in Belfast so I can either use a UK airport (Belfast), or Dublin. In my previous job, we flew business class from the UK to San Fran fairly regularly. It was often £500 - £1000 cheaper to fly Dublin to SFO connecting through London than Belfast -> London -> SFO. Infact, even pricing the London to SFO on the same flight (leaving out the Belfast leg), it was £500 cheaper to start in Dublin.
The price difference was all tax - and lots of it!
The price difference was all tax - and lots of it!