So, Who Exactly is a Member of the Big Six?

I never mentioned neo in my post.  I just wrote Super Seven, my super seven of prog of the post-classic era.  Shadowland nowadays are more darkwave than prog, closer to The Mission than Arena. As for Pallas and Pendragon, never liked them.  I don't recognise hard rock and metal as useful tags anymore, as Arena are in some songs just as heavy as Threshold. 

All six of those bands you mentioned are everything I DO like about Neo Prog. 

Back in the early noughties, I used to personally refer to these as the 'Big 6' of the UK Neo scene..

Marillion - IQ - Pendragon - Arena - Jadis - Pallas

I doubt whether that would have meant anything to anyone else however, as Marillion had partly moved away in style and I'm sure other bands such as Landmarq, Galahad and Grey Lady Down might have had something to say on the matter... 

There were Pink Floyd influences even on the debut album, especially Radioactive Toy. 

The music was progressive from the start, like you say, indie-psychedelia-space rock. Someone mixing genres like that, sounds like prog to me. 

True, nothing to do with neo-prog. 

It's almost as bizarre as Jethro Tull being classed as a Heavy Metal band at the Grammy Awards, although 53% of people on Ranker agreed with Dream Theater being classed as Neo Prog. 

I personally would probably say Jadis but there are a lot of other contenders including Galahad, Abel Ganz, Airbag, Mystery, Sylvan, Satellite, Frost* and on and on. Magenta too even though I initially thought of them as symph prog. However, they are considered neo on here so they get tossed in the bag also. Interestingly enough, It Bites are crossover prog on here even though I used to think of them as neo. They could be a contender also if it wasn't for them being crossover.