So a 'yes' to enforcement is a 'no' to parking without risk.
Today, Xmas Day, you are only at risk of a PCN if you park on white zig-zags (so outside the closed school is ok as they are yellow) or if double parked (which means the entire vehicle is more than 50cm from the kerb, it is always good practice to park parallel to and close to the kerb).
Remember also that the day ends at midnight. Traffic wardens work up until 11pm and then start again at 7am (except they have a small lie-in on Saturdays until 8am and Sundays until 9am).
This guide is only for the London Borough of Barnet. Unless signs say "excluding bank holidays" you will have to check the websites of other boroughs before you set off to see Auntie Mabel.
Have a happy time this festive season.