If you're doing this to save money (rather than a TP-maxxing exercise), then make sure you add in all the extra costs: (i) one-way ticket to your starting point, (ii) extra parking/transport costs to LHR, (iii) hotel if you're stopping overnight.

And make sure you are ok with the risks of doing this - (i) risk of missing your return flight (if doing an out-and-back, or a tight connection at the starting point, (ii) risk of missing the connection at LHR, if connecting straight from Euroland to TLV, (iii) in your case, risk of getting stuck in the Chunnel and everything goes tits up.

It doesn't always make sense to do this, just to save a few pounds. (Remember the stressed, freaking-out guy at MXP, trying to save a few pounds getting to LOS).