My partner's parents are currently in Europe on a Schengen visa, both with Chinese passports.

He has booked two BA flights to get them home on 27th August, but on separate bookings:

BA0751 from Basel - LHR (T5), arriving 07:25
BA0089 from LHR (T5) - Beijing, departing 15:05

They will only have hand luggage so there's no requirement to go landside to pickup bags, but my partner is worried that his parents could be denied boarding in BSL as they don't have a UK visa. He's phoned the BA Gold Exec Club line and been given three different answers - first that it might be ok, then that it wouldn't, then that it would all be fine. The last agent said that they can't connect the two bookings together but that they can put a note on the first one confirming the onward flight from LHR - Beijing,

Can anyone advise on whether this should be an issue? Will they issue both tickets at BSL, or just the first ticket and require them to pickup onward tickets at the transit desk at LHR, or will they not let them fly from BSL at all?

Thanks in advance!
Dan