Ok Patrick I was thinking Hitler did make the bomb ( I know he didn't and was nowhere near ) and it was 1945 the end of the war, in more ways than one. So any equipment that was around up until then. So any British, American, German Italian, Russian I'll even allow Japanese. But you need to make a different vehicle using at least two different vehicles/equipment as Ian says Mad Max style. You don't need to add guns or weapons it could just be used to get around. ( that's what I'll be doing ) Because they were there the 262 and Meteor are allowed.
Ok that seems doable! How about this for a time line :
In 1940 the Germans successfully invade Great Britain.
In early 1941 Germany ratifies & expands its non-agression treaty with the Soviet Union,
On December 8th 1941, Germany declares that Japan is no longer a member of the Tripartite Pact in order to avoid conflict with the Americans after Pearl Harbour.
In February 1942, German espionage agents pass on information about the development of the atom bomb gained from Soviet spies in America. German scientists are immediately set to work on developing a Nazi atomic bomb.
By 1944, America, Germany & Russia all have advanced nuclear programmes in place. Although supposedly at peace with each other, all fear who will be the first to make a viable nuclear weapon.
In January 1945, Hitler strikes Russia with a devastating attack on Moscow, Leningrad & Kiev. Behind the expanding mushroom clouds, the German conventional forces attack deep into Russia.
Forewarned by Soviet Smersh agents & able to flee Moscow from his secure bunker, Stalin orders a counterstrike that leaves Berlin in ruins.
Does that sound like a good starting place? Whoever in the GB could add more to the story in any way they wish according to the build they're doing - it might make it more involving & interesting?