In the recent past, they’ve been on the cusp, never more. In the past 20 or so years, they’ve really served to split the opposition vote. In the 1980s that benefitted the Tories; in 1997 and 2001 that benefitted Labour. We live in interesting times.
At best that makes them accountable to the local party, which is completely different to being accountable to the local electorate. Indeed, one of the strongest objections to the UK first-past-the-post system is that a large number of parliamentary seats are effectively at the disposal of a local party committee - if you can persuade the Tory (or Labour) party to select you as the candidate for X constituency, then you have already crossed the principal hurdle, and are a shoe-in for a parliamentary seat.
And, in any event, it’s rare for a party to deselect a sitting MP; head office will generally lean heavily on them to prevent this.
Look, I lived in Ireland for forty years, and was fairly politically involved. We’re familiar with both systems, since Westminster MPs from Northern Ireland are elected under the Westminster system. There’s general agreement that the single transferable vote system of PR, as used in the Republic and in some elections in the North, requires members to be far, far more responsive to constituents.