Earlier quoted context omitted.
It's the difference between proportional voting vs winner takes it all. In the latter case you can't really hold politicians accountable, as you will have to choose between effectively throwing your vote away or voting for the one opposition candidate, that often will be just as bad. While the UK have some level of representativeness, each circuit has a winner takes it all structure, making change quite hard to achie…
It's the opposite of what you say. Proportional representation isn't accountable because you don't know what coalition you're voting for - coalitions are done in backrooms after the election. Winner takes all is more accountable because the coalitions are done before the election (aka political parties). Parties are made up of different factions and they're agreed before the election.
Deals and bargaining all happen AFTER a party takes power and completely hidden until a government can't pass their own bills like the Labour attempt to reform welfare.
With proportional representation the deals are made in order to form a government, BEFORE it has power, and are between separate political parties.
Sure there may be agreements that are not all made public, but these are much harder to keep in the "backroom".