Build them where?
If the point of the Navy is to keep shipyards busy and their workers voting for you.
Where exactly was a tory government planning to build them /
The Clyde - probably not worth wasting billions trying to buy votes in Scotland !
Ditto Tyneside. You might get a few Ulster unionist votes by building them in Belfast - but they are going to vote for your anyway.
Unless you are planning to fit out Portsmouth to build massive carriers to keep the LibDems happy or you are planning a new shipbuilding industry somewhere in the M25 - you might as well keep the money for tax cuts (or BAe) and forget the carriers.