> the "first past the post" system locks us into 2 parties.

What you say is true, but while we're discussing an article about the UK, it is worth stating, for the avoidance of doubt, that the UK also uses the First Past The Post electoral systems (for its national parliament) and therefore doesn't have a representational seat distribution.

Indeed, that may be part of the reason that such an extreme policy from such an unpopular party is being put forward at all. (The current government won 43.6% of the vote at the last general election, with 67.3% turnout, meaning it had the support of 29.3% of the electorate, but won 56.2% of the seats).