CVF is being put together in Rosyth for two reasons :
1. Its next door to Gordon constituency
2. Harland & Wolff don't have the technical capability to build at that scale anymore (and have a poor recent record with non-upkeep RN/RFA work), Fort Victoria, Argus. They do have the biggest dock in the UK though, 550 x 90m and no bridge to get under.
Biggest carrier in Portsmouth harbour (not Spithead Charlie) in recent years was probably FS Clemenceau (32000 te, 230m long) in the late 90s.
Concur with Navaleye - maintaining a channel suitable for Nimitz would be very expensive. In terms of beam however, they're similar ships. CVF has been as high as 76m, but is down to about 68 now I think. Nimitz is ~ 76m at the flightdeck.