"...but how many who vote for principles and competence also vote Republican?"

Don, there are some...but I'm afraid that many Republicans define "principles and competence" from a different perspective than I...and many Democrats would. A complete exposition of that bold assertion would require quite a long post, but I mean basically that all-too-many conservatives/Republicans begin with a set of 'beliefs', often exemplified by chanted slogans.."no more taxes"-"get the government off our backs"-"stop illegal immigration"-"leave my guns alone"-"stop regulation and let businesses create jobs"-"progressive=fascism"...etc...etc...
   Now, when they many of them are asked to explain 'principles', you get one or more pre-digested slogans which they may or may not understand in the larger context of society....but which they defend as best they can manage as "givens" and not subject to discussion.
   This sort of attitude is much easier to defend and 'run with' than one which allows compromise, re-evaluates issues as circumstances change and tries to see ALL sides of a matter before pronouncing judgment based on....slogans.
Much of the attitudes I refer to are shown in being 'against' stuff, rather than offering detailed plans for reasonable progress. "We don't want to be told what to do...by anyone..especially government!" That attitude often leads to the fingers-in-the-ears "lalalalala" technique of 'discussion', and often to the "so's your Mother" form of reply. Yes, it is phrased a bit better than that, but if you insist on clarity, that's essentially what you often get.

So... are there Republicans who vote "principles and competence"? Sure....and there used to be more of them, but boy, it's hard to get the reasonable ones to stick their noses out these days. Oh, really? Yep
(note: about half... 24... of the more conservative Democrats (blue dogs) were defeated in this election, while only 4 of the 'progressives' lost. Folks were not happy with ANYONE with Democrat by their name...they wanted FARTHER right representatives, based on slogans and distortion.)

I have no idea how this works in the UK, but what little I see of Parliament leads me to believe that many members at least understand their party's positions instead of just shouting slogans..