I tend to agree. If it were say a 7 hour layover, I might be tempted to think "hmm, train each way, waiting time-- I better at least -plan- to heading back to LHR very early in case some of the public transport gets disrupted (inevitable bomb-threats, etc.)" So, I might stay in on a less-than-7(ish) hour layover, right now anyway (factoring in TLV departure as well.)

However, with 12 hours, I think you can easily escape the mind-numbing monotony of the airport and still be back at LHR is -plenty- of time (again, allowing a pretty big time "cushion" on the return time) I did a layover London daytrip last month upon arrival from MEL awaiting a flight to CAI and it was a completely lovely alternative to spending 10 hours in the lounge (and in that case it was Concorde room and was massage-eligible-- took an arrivals lounge back-rub before heading into town and a much-needed T4 foot-rub when I got back! )

In short, w/ 12 hours, go to town (and use "left luggage.)