The "constituency" they represent depends on the PR method used - In Scotland they use a list system, where they elect a representative MP and also a "second choice" list - and the list MPs are proportional for an area rather than the constituency (or that is my understanding). Other countries you have regional blocks of MPs based on the votes in that region, so Yorkshire might have 60 MPs split proportionally with local representation And you are right, there would be 2-3 far right, BNP or equivalent MPS, and there would also be 2-3 far-left communist or SWP MPs, but there would also be around 200 Tory, 150 Labour, 80 Libdem, 50 Green and SNP, etc. But this would mean that no government would ever be able to foist their ideals on the country without the backing of at least some of the other parties - and would have to compromise to get that support. This would certainly have led to a different, less damaging Brexit*. The main parties could also split - both the Tories and Labour are effectively multiple parties within the same party structure with each side wrestling for control. *Many other policies are available.
I think the sad part about this is that from the 50’s to the 90’s this was declining, but the populists have grasped the nettle and managed to reverse the trend. It makes me despair for the future.
There's a really big problem with proportional representation, and its that a huge number of voters don't see the merit of it. Without that, it's not going to get through a referendum. Let alone the stigma now attached to such a thing after the disaster of 2016. The UK seems to like an outright winner. Its endemic in our language and our similes, of contest and victors. It also doesn't help the tories, who have a massive majority and are likely to increase it further with boundary changes in the coming years. So it's not going to be proposed, and if even if it was, I don't see it being voted in. We can look at the 2015 election to see what people thought of shared power. The tories used the libdems as a shield. Took their good ideas. The LibDems got kicked for selling out and the tories are still in power. You can also see tensions when the tories had a small majority and a modest number of rebels siding with the opposition derailed many a government bill. So despite being much fairer, I don't see how you get a path to electoral reform any time soon.
Take a look at Ian Hislop's 'Olden Days' series on BBC. Fascinating but frightening. He shows how the British have always faced the future by looking to the past. Explains the success Johnson has with his jingoism and how the 'Brexit effect' isn't likely to go away.
Wales has 40 constituencies - each constituency has a Westminster MP and a Member in the Senedd. Also Wales is divided into 5 Regions who elect another 20 Members to the Senedd. These Regional MS are voted by a system of Proportional Representation The result of the recent elections - Senedd has 60 seats - 30 Labour - 16 Con -13 Plaid Cymru - 1 Lib Dem. The odd situation (similar in Scotland?) is that some Constituencies e.g. Ynys Mon has a Tory M P in Westminster - a Plaid and a Labour M.P. in the Senedd.
I fully understand and accept the arguments both about extremist parties and local parliamentary representation, but I think there are workarounds for both of these issues. In most proportional systems, I believe there is a 5% threshold for entering parliament. Obviously this is not a complete failsafe but it certainly functions as a layer of protection against the scenario you mention. As for local representation, I believe that the German system takes this into account and has measures in place which try to ensure that MP's broadly represent their local area. I won't claim to know exactly how it works and I know that it's not 100% effective, but even under our current system there is nothing to stop parties parachuting any member in to stand in a constituency, even if they have no connection with the place. This can and does happen, especially in by-elections to get senior members who've lost their seats back into parliament. To be honest, even if you do get a few marginal lunatics in parliament, I still think this is a price worth paying to have a government which broadly represents the views of the people. The current situation, where we have a fairly hard right government with a large majority despite being voted in by a clear minority, is absolutely absurd. In the UK, we seem to have a strange aversion to coalition governments. Personally, I think that more cooperation and less tribalism can only be a good thing.