I'm not personally for a "proper" PR system (or anything with party lists), I think we need STV which is proportional-ish, but still requires candidates to have a decent support in each constituency.So a Tory UKIP Unionist govt would have been better then just a Tory one or
given the vote we just had, on what basis could Labour have governed by the will of the people?
Having said that, under a PR system I think we could have had governments in the following ways:
Tory Coalitions
Tory-UKIP coalition 36.9+12.6 = 49.5%
Tory-Lib Dem coalition with UKIP agreeing to abstain = 36.9+7.9 = 44.8% but the equivalent of 51.33%
Tory-Lib Dem-SNP coalition = 36.9+7.9+4.7 = 49.5% (obviously unlikely with the SNP)
Tory-Lib Dem-Green coalition = 36.9+7.9+3.8 = 48.6%
Labour Coalitions
Labour-Lib Dem-UKIP coalition 30.4+12.6+7.9 = 50.9%
Labour-Lib Dem-??? with UKIP agreeing to abstain
It's important to note that, when people talk about a PR system, most don't want "absolutely perfect" proportionality. That may sound hypocritical, but it helps the larger parties to govern. In Germany, the "Free Democratic Party" (a coalition partner in 2009) took no seats as they fell below the 5% Party List threshold.
So, in virtually all systems, you can bump those numbers up a bit.