Not sure about HS2 but I think crossrail might make a difference.
It's clearly a response to our family travel habit - fly to some distant point in Europe and come home by train. Hurry up BA, I want my avios for last week (and if you could link up with Deutsche Bahn as well i'd really be happy).
I do think it makes some sense as a way of capturing more traffic. Foreign visitors might find it easier/more comfortable to buy Eurostar tickets through BA and people making the Paris - LHR- long haul connection might prefer to travel to London by train which does at least save schlepping out of central Paris.