This is nothing really to be surprised about, you are talking about connecting local flights in a country or area another airline services.

Plenty of airlines form around the world, across all alliances and none will use BA for some UK domestic routes, and indeed others to. Basically using the established operators, with plenty of flights, in the area.

As for a relationship, well they will have a through ticketing one obviously, and probably a baggage interline one - for through ticketed persons.

Seeing flights come up for through ticketing (where you have a connection off another service) really shouldn't surprise people at all.

If you want to see something a little surprising, go to www.swiss.com and do a search for London to Paris.