file a flight plan as its safer for someone to know what your up to.
you wont get a crossing over london unless your in a milti engine aircraft due to the glide clear problem, if you have an engine failure in an SEP.
if your flying the route class G then you dont have to file a flight plan. best thing to do is speak to any LARS service like manston, shoutend and Farnbrough on your route who will inform you of any other traffic.
i dont know if elstree has any nav aids on site, however if your realy in a pickle, you can do a possition fix and navigate by VORs like BKK i believe. if your above the feild with no nav aids and cant see it when your at you MSA, time to divert to southend and land there.
the whole point of an ILS is becasue the pilot knows that as long as they are on it, they are safe from hitting any buildings etc in that funnel to the ground. you cant do an ILS at one airport then fly low level to another one as you will prob fly into the side of a hill or something.
i dont understand your thought process about an MDH at an airport without nav aids. if you are flying into an airport without navaids and are in the white stuff, how would you even know you above it to start an approach with your MDH in mind??