How long is d validity of a letter of invitation from UK?

Hi all,

Pls I want to find out

1. the validity of a letter of invitation sent by a friend from d UK for Visit visa application, i.e. how many months can it still be valid for presentation at the UK embassy from the date of writing it?

2. If I get my visa application submitted, can I be allowed to authorize any other person to collect on my behalf when it’s ready? Pls how do I do this?

Hi guys. I'm still standing by for ur wonderful response.

Rgds.