Not so extraordinary actually. In the vast majority of long distance Palearctic passerine migrants the young usually return to close to their birth site (without looking it up, males closer than females I believe), so unless they have discovered somewhere really promising during their first month/two months out of the nest, when wandering around like a restless teenager, they will aim to stick close to their choice the following summer.
If they do discover somewhere new really promising then chances are it won't be over a body of water as large as the Channel, as this juvenile wandering is at a time when the migratory instinct hasn't really kicked in. They really don't feel the urge to cross it.
When they do arrive in the UK it is while they are making the longer migratory flights, while they are programmed to move long distances towards winter quarters. I was lucky enough to see three last autumn, and the 'perceived wisdom' for these was always 'Scandi birds blown off course'. If they got themselves back on the right route then I have no doubt they'll be heading back for that mythical country of Scandi.
The odd pair that do nest in the U.K. are going to remain just that, unless something extraordinary happens, say a large arrival at the end of the spring migratory period when the urge to travel is almost completely dormant again. Or monstrous easterlies at the end of the nesting period. Or, more likely I suspect, their favoured area becomes unfavourable through global warming. Until that time, as their parents did well where they were raised, then that will do for their genepool.
Tactics are different for things like Cetti's, which, as a more resident species, have a migratory restlessness that can switch off leaving them in good habitat and without having an urge to do an ET and go home.
But all those other long-distance migrants, so close as Calais and yet so far, are doing exactly the same thing as the Bluethroat. Going back to where they planned to. Just be thankful you're in Kent and not Kerry.
As for splitting, having had the pleasure of see Bluethroat races from the U.K. right over to the Orient in just the last twelve months, I enjoy them as they are, a successful species and say 'vive la difference'.
