Why? I mean they are QUIDE LIDERALLY over the sea, isn't it?
It's because Welsh and English teams play in the same league (in part), and it also save confusion should a player move to the Scottish league. All of which could be described as 'abroad' technically depending on which of the home countries you're in and 'overseas' just removes that (admittedly marginal) grey area.
Nah what I meant is that for me, the word 'overseas' somehow implies a far greater distance of water travel than just crossing a few kilometers over the canal.
Obviously I'm not a native speaker, so I guess this is because the direct German counterpart 'Übersee' much more connotes some adventurous travel across oceans, to other continents, etc, and this connotation is activated in my head when I read 'overseas'. I don't think Germans would use the term 'Übersee' for going to Britain.
Also, I suspect that maybe people in Britain have a more distant relation towards continental Europe, than continental Europe has towards Britain, and this varying sense of distance might play a part in the usage of a word like overseas/'Übersee-'.