Thanks everyone for the suggestions. A train from Aberdeen to Heathrow would be 7-8 hours, so not ideal for my parents... Flying from Heathrow makes a lot of sense though, perhaps it's best to just bite the bullet and book the tickets separately, perhaps with a long transit time to reduce the stress of collecting the bags and going through security again...

It's a shame SA don't use BA for a partner anymore - this used to be much simpler!