I never add a tip when someone has done it for me, especially in addition to a "service charge".

Although it's a bit of a shame, because you wonder how much the person delivering the food is actually getting (chances are they're not making very much to begin with) and how much is going to the hotel. There is talk of making a law in the UK disclosing how much of the tip is actually going to the server, but as of right now it's just talk.