Instead they fitted a hybrid of a US W59 fusion secondary, triggered by a new British primary based on a Cleo boosted-fission device tested in Nevada as PAMPAS and TENDRAC.
Variants of this basic design were used or intended for several other delivery systems, including the WE.177 bomb, UK Skybolt ALBM, and the Blue Water SSM (a tactical battlefield missile), and several others.
Fission of the depleted uranium casing can generate a significant proportion of total weapon yield, and compared to other fissile material U-238 is cheap and plentiful as a waste product from HEU manufacture.
Since the Reggie secondary manufactured in the UK from British-owned materials was a copy of the US W59, by implication, the W59 thermonuclear weapon deployed in some Minuteman I ICBMs also used the fission-fusion-fission process.
When retired, the Reggie fusion secondary was recycled and reused in its successor, the Chevaline warhead, matched to a new boosted-fission trigger codenamed Harriet.