How about Blackpool? Plenty of places to stay if you wanted, lots of diverse entertainment 24 hours a day & Bloomfield road wouldn't look half empty on the telly like Cardiff! Would have to be in the summer of course.
I like the French option too, although that would be a issue with some due to the cost.

Saying that, it would be like a home game, 3k-4k Quins fans would be an added bonus to the crowds, it's a hell of a lot easier for Catalans fans and the transport links could be better for London but they are still very good compared to the rest of the UK.

There is space around the stadium for activities and not many pubs close to the stadium (keeps fans around the stadium).

It could also help grow Quins crowds.

Make the Magic weekend an International event. Make it an annual one off GB Lions test against the Aussies or Kiwis and put it on at the Millenium or a venue in London. (Emirates?)
Could have France verus Ireland as a warm if you want to keep the multiple game thing going but I don't think that really makes much difference.

It will be far more attractive to the neutral, get more publicity in that area and still provide a jolly up for all rugby fans. Make sure no other games are played that weekend.

This could be the curtain raiser to the season or mid-season.

Profits from the game would then be split between the clubs giving an incentive for them to ensure it sells out!