Can you say 'Tornado with fixed wings'? 
Already one 'normal' engine (Avon, Spey, RB.199, 'pass-though' Pegasus) gives a better thrust-to-weight ratio, compared with two Adours. The RB.199 is, from the British point of view, probably the best choice - logistics are there wrt. Tornado, it is a more modern engine, light and of modest size. More than 600 kg/1300 lbs can be saved with RB.199.
Avon is a tad too late, unless the work is started rather early (early 1970s at latest). Otherwise it is a mature engine, that might be easier to get the export permission. Not too big nor long. Fuel consumption will go up noticeably vs. two Adours.
With reheated Spey on board, a major power increase is to be had. It will take a more substantial change to the fuselage, though. Increased weight will require bigger wing and stronger undercarriage.
Installation of the non-afterburning Spey might be much easier to do; low power, though.
Pegasus is interesting - bulky it was, but it was also powerful.
