for reasons already stated I believe it is legal and normal to plan an approach to LHR 27L/R OEI;
however, if you consider the Kegworth BMI 737 incident, (where a planned OEI approach was made to a 'rural' airport rather than any other of the options which would have included LHR) I can see why the question would be asked. I can only say that before I flew over a congested area I might ensure the operating engine was 'normal' with a period of level flight before commiting myself.
I have absolutely no wish to get into a debate about Kegworth, it was simply an example of an OEI approach which did not turn out as sucessfully as the crew intended.