Depends on the airport. Some are more jobsworth than others. The safest bet. Letter of the law type thing. Shove any purchases into your hand luggage so that you still only have one bag. They will tell you off for multiple bags. They rarely check for extra weight once you are in departures. Unless it's plainly obvious that your bag is so heavy you can hardly carry it lol. When I online checked in for Stansted I never once went anywhere near a scales as I bypassed the check in desk. Tampere in Finland though weighed everyone before you entered departures no matter how you had checked in.