Couldn't Barrow do it, I know they build ships that sink, but would they have the infrastructure and also more importantly the deep water channel to accept it.
Also as they built sections at Portsmouth wouldn't they be better equipped to complete them, plus they just dredged the channel for it.
And Courtney, do be serious, you do not think the MOD would use something as modern and up to date as Windows 10, surely XP or one of the free systems would be a cheaper option.