They don't tend to use green uncooked prawns in a prawn cocktail, they use the REDish pre cooked ones.
My nurse told me it was FINE to eat them as long as they aren't bought already prepeeled and as long as they come fresh from the fridge and aren't left out.
Ya know on Christmas day how sometimes you have a large bowl of prawns on the table that everyone eats from, well thats a no no, yours has to come fresh from the fridge and eaten straight away.
Enjoy, I love prawns and we will be buying some for Christmas.