I believe that some regular operators into LHR have (or, at least, have had in the past) provision in their OPS manual to designate the second runway as the alternate in certain circumstances. As others point out, it doesn't seem very sensible.
Those who take the view that if one runway becomes blocked they'll just land on the other - and I've had the discussion with a senior level management pilot from a major carrier - obviously have never seen the chaos (albeit controlled) that ensues in a busy terminal area when not all the expected runways are available. And that's before considering the common failure modes that will make both runways unavailable - like JB's example of a fire in ATC. Still, the JARs permit it so it must be OK.....