It's not necessary, in the almost 10y I was at my previous address I only used (other than the odd web form with mandatory 'address line 2' or whatever) flat nu

It's not necessary, in the almost 10y I was at my previous address I only used (other than the odd web form with mandatory 'address line 2' or whatever) flat number & postcode. It's not a weird niche blogging exercise, it's just how it works. It's usual to be a bit more redundant on the front of an envelope/parcel for whatever reason, but the normal form for a return address is '12/AB3 4CD'. There's nothing magic about returning post that makes a shorter address start working! (It would have a postmark additionally by then, but that's not extra information - the postcode is already more specific, being on just one of that office's many routes).