I can add some information for ASR.390.
A draft was written in March 1967 for a Multi-Purpose Training Aircraft to replace the Varsity. It was to train pilots, navigators, Air Electronics Officers and Air Engineers. An “off the shelf” aircraft was desired to try and avoid development costs but a turboprop was specified. The first aircraft was required in service by June 1972.
The ASR called for a normal crew of 2 in cockpit and capacity for 7 staff/students in the main compartment.
Pilot Role – 2 pilots and removable utility rearward facing seats in main compartment for supernumerary crew if required.
Navigator Role – 1 pilot, 1 Staff Navigator and 4 students
AEO/ Air Engineer Role – 1 pilot, 1 Staff Navigator, 1 Staff Engineer, 1 Staff AEO, 2 student AEOs and 2 student engineers
The maximum speed range was 250kts TAS at SL to 0.6M at 25,000ft. Fuel capacity was to be provided for: 1,300nm stage length with full fuel reserves and payload; two 2hr sorties (1hr general handling at 10,000ft and 1hr local circuit flying below 2,000ft) without refuelling plus fuel reserves; air endurance of 6hrs.
The Air Ministry had the Handley Page Jetstream in mind for the role. The Ministry of Technology (MinTech) suggested the HSA Dominie, Argosy or Avro 748 as possible alternatives. They were sceptical development would be cheap, the equipment integration costs would require development funding. They estimated the Jetstream's AUW would have to be increased from 12,500lb, beyond the 14,000lb suggested for the American C-10A variant, to about 15,500lb, and they thought external fuel tanks might be required.
There was argument over whether a turboprop offered sufficient performance for navigator training and the recent introduction of the Dominie for the navigation training role seemed to meet or exceed ASR.390 in everything but the capacity for 7 in the cabin. In the event ASR.390 was cancelled and it seems Handley Page were never asked to make formal proposals.
Eventually in February 1972 the RAF did indeed order 26 Jetstream 201s as multi-engine trainers as the Jetstream T.1 to OR.398.
Source: AVIA 65/2197