Of course you are right CC that the people who are chosen as candidates for parliament are chosen by the constituency party and the card-carriers and not be the general public at large.
I don't think that it would be workable that EVERYBODY should get a say in who is presented for each of the party candidatures as that would just be too clumsy and expensive to do even IF the great majority of the electorate actually gave a damn about who representated them, which sadly, many of them don't.
All or most politics in Britain today are party politics and the voters also separate down party lines too and that is why most of the candidiates run under particular party banners. Of course a few of the candidates are also independents or from minority parties and if we want to we can vote for them too if we wish.
I think that we already have the situation where each MP only sits for the duration of the parliament as after parliament is dissolved they have to re-apply for candidacy each time. ( yes, a lot of them are an automatic shoe-in as far as the constituency party are concerned and sadly there is little anyone outside the party can do about that situation ).