This was interesting.

] Express messeges were sent by rich private messengers.A public service - enabling ordinary individuals to send urgent messages only began in 1891(!)It was ferociously expensive,BTW, costing 2 d for the first mile,3d for the second and a shilling for every other mile.

Now I have to ask what the d in "2d" and "3d" stands for.
I suspected that it would have been expensive, but it was used so frequently in P&P.
] So- Colonel Forster's message to the Bennets would probably have been delivered by a servant of his or a serviceman who could ride.The military were used to sending messages for tactial purposes, according to my source at the GPO.
This seems very logical too.
Thank you once again for taking time to find an answer. Give your source at the GPO a big thank you as well. :)