"@Nigel from New Zealand: If you only want the WiFi then I can see that you are going to buy something just to ensure you are a customer, so in that sense it wouldn't be free, but I go in Wetherspoons anyway and I don't pay any more for using the WiFi, so for me it is free.
A lot of tourists go to McDog because it is cheap and you know roughly what you are going to get there (and some people actually like their stuff), so it helps with holiday budgets and once you are in there you can log on to The Cloud for nothing. Students use McDog for the same reason and because of that there are some which look like internet cafes at times. If you are going in there for the food then it doesn't cost any extra for the WiFi.
In St Pancras International station you just log on. No need to buy anything, so that one really is free.