There was considerable interest in reforming the voting system back in 2010-11 until we found out what was being offered and that there was no choice about what type of PR we could vote for. Then it ceased to be discussed, because what we were being offered (in a first-past-the-post vote!) was if possible worse than first-past-the-post, and was voted down in the referendum. It seems likely that this was careful and deliberate policy by the party in power when it was proposed (Gordon Brown's.) (Cameron should have been forcibly prevented from holding referenda, but unfortunately getting his own way in that one at the expense of Nick Clegg, and in the one about Scottish independence, made him think they were a good idea. The next one definitely was not, from his point of view.)
If the states had a choice to copy Maine and Nebraska or not, what would happen is that the Blue states would allocate Electoral college votes proportionately, thus reducing the total Democratic representation by adding a few Republicans from those states, and the Red states would not, thus retaining the same level of Republican representation from those states that they have now. It would need to be universally applied or not at all.