Mason (British Fighters) said they started out the design with less sweep but kept increasing it to cut drag no mention that a delta was ever considered - and it was only meant as a high speed test aircraft to start with.
Gunston (Early Supersonic Fighters of the West)has about 30 pages on the Lightning - According to him (and Beaumont) the Brits were impressed that as standard F-86 could break the sound barrier with swept wings and that's what they put on the P1. There was enormous amounts of heat and arguments about the tail plane but the wing? Hardly mentioned. It worked.