The decision may have been influenced by the debate about the setting of a high or low tail plane.
The high-tail, delta wing Javelin had many low speed problems. Many of the theorists (RAE) backed the high tail option for supersonic aircraft, but Petter resisted.
The Short SB5 research aircraft evaluated both high and low set tailplanes at low speed; the fears of the low set tail were unfounded.
The notched (split) delta platform with separate low set tailplane had advantages over a pure delta.
https://en.wikipedia.org/wiki/Short_SB.5
https://en.wikipedia.org/wiki/English_Electric_Lightning