I heard that Heathrow Airport has a 2 bag limit when going through security. How strictly do they enforce this? If i have my carry-on and personal item and a plastic shopping bag/carrier bag is that an issue? Is it any different going through connections?
I've never seen security caring about the number of bags when your going through security. As a matter of fact, they probably not even aware whose bag is it, unless they see something suspicious in one, and they need to do a manual check in front of the owner.
On the other hand, can you put your shopping bag into your carry on?
However, it might be a problem at the gate, some airlines might be more strict than others with the number of hand baggage items.