Wasn't the dH 121 a completely different aeroplane? The aeroplane dH wanted to build before BEA mandated only 90 seats? It was to have the proposed RR Medway engine, but when downsized to 90 seats and given a 'T' tail the Medway wasn't required and Speys were fitted.
The Trident 3 was 'dH 121 - sized' and could have done with the Medway, hence its boost engine to augment the 3 Speys in 'hot and high' T/Os.