Maybe it was not a joke! Having worked extensivly in/offshore Scotland I many time heard a reply to the question "Who do you support", and irrespective of the sport the answer from a Scot was often "Scotland, and whoever England are playing".
Pretty common attitude in Scotland. It does run to different depths in different people - and with varying degrees of 'seriousness'.
But you might reflect on why.
It used to be the World Cup would come along and the London based (maybe a bit more Salford based these days?) media would focus on the England team to the extent that even in Scotland somebody not very interested in football would probably be able to name more English players than Scottish.
These days I think it's not so bad ...... because Scotland doesn't qualify for the World Cup finals.
And Rugby isn't much better at times.
There have been complaints about BBC Commonwealth Games coverage over-focusing on English athletes.
Modern professional sport with big sponsorship money makes things worse. The branding around the English national teams and the likes of the English Premier League mean that in terms of other branding and publicity there is more resource available to them and more publicity.
The stupid thing about all this is that there are separate football leagues and that the UK competes as a non-united set of home nations in some events. Where you have one country with 90% of the population and therefore bigger sponsorship and higher numbers of sports people and that dominates the media then it's just asking for friction when it comes to dealing with Wales, NI, and Scotland. There are additional effects such as the relative invisibility of Scottish or Welsh league teams in the Champions League. So you have prime time football shows that are pumped up in the UK media that are likely to be disproportionately focused on English Premier league. Both Celtic and Rangers are masssive football clubs - the grounds and the attendances are bigger than many English Premier League clubs. But the money available to them is a lot less within the Scottish league.
There should really be a single British football league and at international events the UK should always compete as a single entity under a single flag.
If that had been in place a long time ago then the UK would be more culturally united.