Help me find the appropriate stamps for the UK

[morwesong]

This isn't my post, but I figured it would be appropriate to ask my question here.

I am in the US sending to the UK. Last time I was in the UK I bought a bunch of first class stamps. About how many should I put on the return envelope? I include a LOR, return envelope, and sometimes a picture. I'm not entirely sure how much it weighs, but it isn't much.