How strict are restrictions when it comes to multi-stop flights?

Q. My sister and I have flights booked from Dublin to San Francisco via Heathrow and back from Vancouver to Dublin, again via Heathrow. Although my sister wants to join the flight at Dublin, it would be much more convenient for me to join/leave the flights at Heathrow. Is that possible? Or are there likely to be airline rules that will stop me? Name withheld

A. The journey from San Francisco to Vancouver is spectacular, with intense natural beauty complementing fascinating communities. You will certainly have to go to Dublin first. If you don't turn up for the first leg of a multi-stop flight, the whole trip is likely to be automatically cancelled without refund.

That may seem counter-intuitive. After all, you would be saving the airline the trouble of flying you between Dublin and Heathrow. But look at it from the airline's point of view. High-quality, non-stop flights from Heathrow to San Francisco and back from Vancouver command a premium – fares are significantly lower if you connect through a European or North American hub.

For the Irish market, though, the airlines are in very different circumstances. There is tough competition against a range of carriers who sell tickets to San Francisco/Vancouver at good prices via hubs in North America, with excellent connections and journey times. Airlines who serve the cities from Heathrow – which, from an Irish perspective, is in the wrong direction – must offer very tempting fares to lure Irish passengers. These prices are significantly lower than those charged from London.

In addition, travellers who are merely in transit via Heathrow avoid Air Passenger Duty, which UK-originating passengers must pay. While the whole trip will be off if you don't board the outbound flight in Dublin, coming home is different. You must go all the way to the Irish capital and your bags will be tagged through to there. But some people have broken the rules on the return leg by taking cabin baggage only and simply following the "exit" signs at Heathrow.