Easy - you explain that the three leagues are separate entities, and that riders are not salaried employees, but are self employed and make agreements with teams from season to season, which can include (within a framework of limitations which apply to teams in the respective leagues) with teams in two of the three UK leagues, and indeed also make similar arrangements with teams in Polish, Swedish and Danish leagues.
You'd add that a large proportion of riders, in doing this, seek to maximise earnings from Speedway during the months from March to October because, for whatever reasons, their income from November to February is restricted. You'd also mention that many riders employ mechanics, lease vans, and rent workshops, and these are not paid for by the clubs they ride for.
You'd also mention that not all riders have the same set-up as regards income and employment, for example there's one who drives an HGV all year and who just rides in one UK league and not for a Polish/Swedish/Danish club.
At that point I'd invite questions, respond to them, then say goodnight and leave before any question about Guest Riders was raised!!