… The final polling average showed conservatives ahead by 6.4 percentage points. In fact, Conservatives should wind up winning the popular vote by 2 to 3 percentage points. That means the polling average will have been off by about 4 percentage points. …
While a 4-point error would be fairly large in the context of a U.S. presidential election, it’s completely normal in the case of the U.K. On average in U.K. elections since World War II, the final set of polls have missed the Conservative-Labour margin by 3.9 percentage points, almost exactly in line with this year’s error. CONT.
Harry Enten & Nate Silver, FiveThirtyEight