So Labour claimed to be in favour of PR,

No they didn't. They promised a referendum on AV, "Alternative Vote" - but though that has significant advantages over what we've got, and would eliminate the need to go in for Tactical Voting, it is not PR.

In fact, as with the existing system, it would be quite possible for a party to get millions of votes - even more than other parties, and not get a single seat.

Look at that link I gave to the Electoral Reform Society, which explains things like that in detail.