681 was indeed barking mad! For STOL it would (probably) have had 4 x 18290lb thrust BS100 engines under development for P.1154, but for VTOL those would have been supplemented by 2 wing pods each fitted with 9 x 11000lb thrust lift engines....
Fortunately, but to the detriment of the British aircraft industry, the 681 was cancelled in favour of the C-130.