I will guess that you need to oil the meat - or wrap it in bacon to hold some moisture, so it does not turn out too dry....
London broil is a cut that needs to be watched to moisture content escape. It is partially because it is lean. But, then - Tri-Tip, trimmed up for a cook - is even MORE lean right?
So- what is the reason for the potential "shoe leather London broil?.......
Look closely at the direction of the muscle fibers. In a Tri-Tip - they run along it's length, and are fairly uniform - right?
But, with the London Broil......they are more vertical, but can also be ..."confuso" - running in all directions in one cut of meat.
To me - what all of this means is that the LEAN cuts of meat that I can predict the tenderness of ( without augmenting with applied oils, fats or other moisturizers) will have longer continuous muscle fibers, that help to trap moisture and hold it within the meat.
Just my experience...your milage may vary....but - I think it will be similar