Wasps first professional game at QPR's Loftus Road stadium was the first and only time police have been at Wasps game, they arrived by the bus-load including ranks of armoured police horses. The internal areas of the stands were padlocked and guarded and the the stewards (they weren't rugby stewards, but the football ones) were worried about fans drinking alcohol while watching the game. The following week there were no police, the gates between sections of the ground were open and no guards, no need for it.
At the beginning of recent Premiership seasons 60k+ supporters of Wasps, Harlequins, L.Irish and Saracens have all come together in Twickenham's pubs/restaurants from 10am till closing, sitting together through the two games played at Twickers for the London double header. During the day the police just need to man the road-crossings and direct the crowds onto/off trains at the rail station, everyday crowd control basically.
That may not be happening any more though, with Wasps no longer being a Buckinghamshire (Londonish) side, or perhaps more importantly, not wanting to use it as a home game. Though this year Sarries used it as a home game, so perhaps it'll continue as a historical thing, it's a nice bang to start the season.