I've been lucky. Three of the four places I've lived have been blessed with swallowtail butterflies. Admittedly, in Norfolk they are not easy to see, you are more likely to see a cat swimming. I saw one swallowtail, at a distance, whilst I lived there and I lived and worked on the Broads. However, the Norfolk Wildlife Trust feels the future "looks brighter" for the swallowtail with cutting of reed and sedge allowing other plants to thrive including milk parsley which seems to be the exclusive food of the British subspecies. Being tied to one plant food is thought to restrict their ability to spread to the rest of the UK.
The continental subspecies is said to be much more liberal with its choice of larval food, as long as its an umbellifer. I'm sure this is true, but in our garden, where the Brittany photos were taken, dill and fennel, when available is definitely the firm favourite. This doesn't surprise me as fresh dill is amazing, any attempt at preservation making it ordinary.
We often see swallowtails at the coast -we do spend a lot of time there. Wild fennel does well at the coast providing a food source for the larvae.
In Crete we planted fennel and dill, as we do in Brittany, to lure the swallowtails into our garden. I appreciate that all insects are important, but some simply bring joy.
I would imagine that the absence of swallowtails in most of the UK has something to do with the last ice age and the Channel preventing recolonisation. The continental subspecies has been spotted in the UK, presumably storm-blown. Perhaps UK residents should plant some dill just in case.
With a flight period coinciding with our season, there's a good chance of spotting a swallowtail on the Brittany coast path.