Just to summarise my understanding of your position, so as not to be thought to be misrepresenting it: you prefer FPTP because the government you vote for, based on their manifesto, is what you get (if they win); whereas with PR, the horse-trading after the election means it's not clear what bits of which manifesto will actually see the light of day. Apologies if that's an oversimplification.
My problem with that is that broken manifesto promises are pretty much a given anyway, so you don't really know what you're voting for. And more than that, the parties have, in their more underhand moments, taken to burying some of their more dubious objectives deep in the small print of the manifesto where they'll probably escape scrutiny. Assuming the electorate troubles itself enough to read the manifestos in the first place.
Manifesto promises get thrown under the bus because 'events, dear boy' or because Treasury won't fund them, or any number of plausible reasons. So all you can really rely on is a subjective assessment of the ideological stance of the parties as a guide. So you don't really, deep down, know with any degree of certainty, what specifics you are voting for, and post-electoral horse trading, aka Parliamentary debate and process, is as likely to control what you actually get as it would be under PR.