Cromwell VIII, Comet, Achilles (M10 with 17 pounder), Archer (17 pounder tank destroyer), Churchill VIII, AEC Armored Car with 75mm gun (used in 1944-45), Valentine VIII? would all be good additions. Me personally im more interested in what a nation crewed for AFVs and aircraft and not so much who manufactured them but there are a number of "built in the UK" GVs that certainly deserve some attention and would be great to see.
Of all those I think I would pick the Cromwell VIII as the first. It was used in quantity in the NWE campaign from Normandy untill VE Day.