Why not look at Bristol for hotel room as the train station is not far off the centre and Cardiff station is just minutes away from the stadium and late trains till around 10pm , plus your away from the rip off hotels so may save some of the aggro .
Ps or save your money and go to the Marmande Long Track championship in France on the 13th of July lovely weather , great racing on a real racers track and i only live 35 miles Cardiff and once was enough hated it that was the British GP very over rated .