have aircraft gone missing in this area before? YES
in fact, during the fossett search planes that have been missing for over 30 years are being found (6 so far).
unless there is secret air force radar involved, regular ATC radar is useless for the altitudes he was likely flying.
I realize the UK doesn't really have mountains...so unless you take a look, you don't have a clue...the US Miltary uses the area nearby to train some pilots for work in Afghanistan. some areas nearby are marked on the chart as "wilderness".