If your arriving fairly early and have no club colours on, you will be able to drink anywhere. Mind you, when Northampton came to us a few weeks back, their fans were in the Wetherspoons pubs with their shirts on. I do sense though that the police presence will be slightly 'higher' for this game.
Arrive with everyone else and you could find that you are told where you are drinking :icon_bigg
The Counting House at Freemans Common (just by the Morrisons supermarket) is the ideal choice IMO. Close to the ground, Food on offer, Good beer selection, Several sport screens, large premises .....
You may see another pub just across the car park called 'Local Hero' ... this is a home fans only pub.
There is a Nando's on this site if you fancy something a bit different to 'pub grub'