Postal ballot: what happens if you post envelope A?

[puddeekat]

My son voted for the first time this week, by post, but didn't put envelope A inside envelope B. Instead, he posted envelope A. Is it likely to have got there anyway, or would it be invalid?

Invalid

I’ve done exactly the same thing what a fool I am. I posted envelope b separately hoping it would still be valid but I don’t think it will be