I was privileged to listen to the whole thing "live" , I offered to help when I initially heard ATC on 121.5 failing to establish contact, but they said the aircraft they were trying to contact was close to Lydd, so well within the range of their transmitters (London)
It does indeed seem incredible that they could just blunder into the London TMA without contact, as if that were the normal way, I can only hope for them a total radio failure (unlikely as that may be ) as it is difficult to fathom how they could have imagined that everything was OK. If they were flying in the area reported, I think the chance to shoot them down (as threatened) must have been lost quite early in proceedings, as most of their route after coasting in was over populated/heavily populated areas.
Having spent some time flying in a Baltic (previously Soviet) country, I am open to believing that the mentality required for this to happen could well be present on the flight deck.
I guess, if it actually managed to get as far as Heathrow (as alleged here) serious questions will be asked in the house of "Her Majesties Govt" as, if something sinister was planned, that (with respect) would be a bit too late.
If you look at the following two links, you can see why (and who to blame) for this "edginess" from the R.A.F.
http://www.bbc.com/news/uk-england-29823148
http://www.bbc.com/news/world-europe-29825890