I'd second the advice re Upfield Farm. (My aircraft's based there) Approx 30 mins drive to the Millenium stadium from there. No lights at the airfield, but with the extra hour of daylight, you should just get back in time. Cardiff airport about the same time to drive, and though there is a train service, I doubt it'll be much quicker.
If you do opt for Cardiff, ask to be "handled" by the flying club on the south side - the landing fee will then be ~£18. Dont worry about joining procedures, they're pretty simple, and ATC are great. Well worth checking that the club will still be open at your planned departure time though, you'd do very well to get back to your aircraft by 6.30pm after a footy match)