I've never had to use it - I could envisage its being difficult to prove the value of what was in the envelope/parcel, though. Fortunately, the postal service has, in general, been very reliable over the last few years, with the occasional disaster
https://www.theguardian.com/busines...se-across-ukFortunately, I'm in south-west, not south-east, London!
Dedicated post offices of the kind one finds in France are less and less common here - often part of small supermarkets now, as my local one is.