Are British posties LEGALLY obliged to use a shoulder sack to carry mail between addresses - re security - rain protection...?

Legal ? That would mean a law passed through the House Of Lords ---- for a post bag!

It is the policy of the Post Office not the Government.

I wanted to get all the question into the first box. Sounds ok - but I wouldn't mind a link to their rules - must be on the Web somewhere but I couldn't find anything. (All sorts of valuable things are sent through the post, share certificates, jewels etc. Albeit 'signed-for & insured. Some kind of security is surely necessary, even a big, heavy red bag - which also keeps everything dry. I've had rain-wet mail with the writing in a letter inside running..)

they dont use a sack they use a trolley