Although drones clearly make good surveillance platforms, I'm not convinced that propagation delay when associated with fine adjustment of SIGINT equipment, particularly where accurate direction-of-arrival is needed, makes them 'ideal' for such a role.
Neither Comet- nor Boeing 707-derived platforms are in their first flush of youth. Both have 4 engines because they needed them back in the 1950s, with all the associated maintenance requirements.
An aircraft of roughly the same physical size as the VC10, yet which has 2/3 the burn rate is becoming available. The Luftwaffe are replacing their VVIP A310s with A340s; the A310 would have all the electrical power and space needed for the mission requirements of the role. Same fuselage cross section as an A330 and plenty of room for mission specialists and their equipment.
Price would probably be right too - and no worries about 't bungling Baron Waste o' Space trying to modify a 60 year old aeroplane design.