Well, many tourists need a visa to connect via the UK. I avoid travel through Heathrow because of it. (No, not as a protest; I'm usually traveling with someone not covered by a reciprocal visa and it's just easier that way.)
http://www.ukvisas.gov.uk/en/howtoapply/infs/inf20transit#23...
Even though they say that entry is "at the Immigration Officer’s discretion and there is no guaranteed right of entry", I've used this concession twice (I'm a citizen of Armenia) with no problems, and have never heard of anyone being refused entry.
The first time I used the concession was when I was flying NYC -> Heathrow (arriving at 2am), followed by Gatwick -> Prague (departing at 10pm). Not only did this allow me to take advantage of a cheap EasyJet fare, but I managed to fit in a full day of sightseeing in London into the 24 hour allowance.