There is only someone at the barrier to take money immediately after the game. Otherwise you need to buy a token from the bar. It's £1 for parking if you arrive early (Before 2pm I think) or £5 if you arrive later. I find it easier to buy tickets online, you can print them at home or just show the scanner the image on your smartphone.
Bar in the East stand does decent fresh cooked hot food that you order from the bar and collect from the kitchen, the west stand is served by a couple of food concessions selling Hotdogs\pasties . All the bars sell baguettes as far as I'm aware.
The East Stand is has plenty of sofas and chairs and is geared towards having a drink and something to eat before\after the match (Drinks are served in glasses unless you ask). The two bars in the West stand have bar stools and are more geared towards standing up and having a pint during the match.
I'm guessing that with Wigan bringing a few all the Bars are going to be very busy. If it was a football match then the East bar would be home supporters only but I don't know how they will play it for rugby. Possibly refuse entry once it gets too full.
There are a couple of places within 10-20min walk. Regency Club (Bar & Grill) at Queensbury Tube (15min walk), Moranos (Bar) at Canons Park Tube (10min walk) and The Wishing Well (Pub) in Edgware (20min walk). Some go to the Wetherspoons in Stanmore which is one tube stop away from Canons park.
Anyway hope you have a good journey.
I come in on the M1. I've found the best way is to come off at Junction 4, turn right at the first roundabout, go over the next roundabout and then left at the lights (Next to Stanmore Tube). Then follow the road (Marsh Lane) until you reach another roundabout and turn left into Taunton Way. Hive is on the left after you go over the railway bridge.