I would use a cheap contract white emulsion (watered down as you mentioned) for the first coat, then give it 1 or 2 coats of Light Space. Light Space is quite expensive and it seems a shame to use it as a mist/piss coat.
I use Wickes Pro paints for most things, it is a bit more expensive than their budget contract paint but I like the coverage. You also get "paint for new plaster" but I think it may be a bit gimmicky?

I have used L&S a couple of times and it is strange, it is matt but shines like gloss (which makes no sense but you will see what I mean when it is on). I used it on communal stair wells etc where it can be a bit dark and dingy, it helped to brighten it up a bit.
Value for money, not so sure, but then again I don't think any Dulux paint is VFM (I am a tight git  )

Wagner Project Pro 117. Watched him set up and use today. Put the supply tube into a 10 litre bucket and away he went...Looked awesome. Gotta 'mask up' and also spend a bit o time covering windows etc but as theres no skirtings/ architraves to dodge then It looks awesome. The coverage was great compared to my roller but thats because it does use the paint up quicly but thats good coverage 4 ya... Non patchy and great finish...Time factor is un believable. Gonna charge me for £20 per day. Final coat may be tricky where colours meet but if you cut in a bit larger area then spray coverage is pretty controllable/ direction
keep ya posted.  I will finish this bl..dy self build this year

Used it and Wow  .  Positives are SPEED, and Finish. 1 and half days to do Mist coat on whole of Thirteen roomed, 4 bed  (3 Large rooms and large open Hallway) House. And that includes Masking lots of things up and moving things about to clear rooms.
1 coat is enough for under/ mist coat!! Good coverage!!
Negatives- Room must be empty so you can move freely so as to avoid overspray, runs etc.
Spray particles do go everwhare especialy when doing ceilings.
Also difficult to mask up because draught from spray disturbs it. Got no skirtings, architraves downstairs yet so awesome.
Uses paint at rapid rate so not sure how it compares to roller. Used aprox 90 Litres on undercoat. Must have a rethink about final coats before buying it (Cant believe what price paint is per litre, and wonder how other cheaper brands compare to Dulux??? )

Thanks for help/ advice on this. Have posted another one related to paint prices ..