It's generally a good idea to get a good night's sleep before travel. If you can't manage to do that, try not to walk for (literally - aside from sitting down on the Tube a few times) for seven straight hours.
If you can't seem to do even that simple act, then try NOT to get a bed in a hostel that plays techno at all hours. Just try that. For additional happiness, you can nix the singing Italian girls, too.
Or, you can travel my way, eat puffed rice for dinner, and wait with every breath of your body for it to be past 9.00 so that you can drag your tired, aching body to bed.
I'm not blaming London, or even the hostel. My fault entirely (but the Italians don't help)