You can have 16 segments so you have two more.

I don't know how many times I have to say this (and I apologise to others for the emphasis) but...

THE BILLING ADDRESS OR COUNTRY OF ISSUE OF THE CREDIT CARD MAKES NO DIFFERENCE WHATSOEVER TO THE PRICE OF THE TICKET!

Itineraries starting in the UK and bought from a UK office will be billed the ex-UK price in GBP. Period.