The numbers in the picture below is confusing to me, I have lived in different flats in London and never came across a similar system. Do they refer to the floor? the flat number?
Recently I have sent a parcel to Glasgow from Mail Boxes Etc office and it came back because the address is so confusing that they themselves have got it wrong even when they send hundreds of parcels everyday.
Add to that that various high-profile companies/websites like Argos reject the "/" when you enter the address manually, so you are either lucky to have the address already in their database or you need to substitute the "/" with an equivalent of letters. I have used "of" three times so far, the postman hasn't understand that twice.