Normally in the past it has simply been a case of Labour + Liberal (Democrats). Although these days one could probably add in other parties to the mix, like Green and maybe say SNP, Plaid Cyrmu, etc.
So, in 18/19 general elections, progressive parties received the largest vote share -- although there was a Conservative government in 9/19 cases and a Conservative-led government in 1 case. So this confirms in numbers that the voting system favours the Conservatives.
It is rare for the progressive parties to even go below 50% - e.g. it happened in 2015 presumably because (a) the LibDems were punished for going into a coalition with the Conservatives and (b) UKIP's vote-share rose lots (and it seems that UKIP has a mix of right-left-conservative voters).
EDIT: Andy requested the seat info too, so here it is: