Warm Roof upgrade - or unnecessary complication?

[harrythefurrysquid]

My 1970 house has Dormer Windows front and rear (similar house nearby), so about half the ceiling is under flat roof. They are uninsulated as far as I can tell - about 180mm air gap vented into the loft. Looking to fix that given gas prices - boiler has to work quite hard to keep it warm! Way out of my DIY limits, so it'll be a pro.

Seems like it's a choice between:

1. bunging some 100mm celotex between the joists from the outside and re-cover - leaving it as a cold roof with cold bridging across the joists and no vapour barrier above the plasterboard
2. building it up as a warm roof - new decking, vapour barrier, 150mm insulation and either felt or grp fibreglass on top (I accept this will result in a step unless I can get the neighbours to join in too)

Option 2 I'm assuming will be significantly more expensive - and I found a few roofers I've talked to so far seemed to be more comfortable settling for Option 1.

Quoth one such builder based on pics:

To be honest it's gonna be a lot more expensive the way you want it for the exact same outcome but if that's what you would like it's fine we can quote for that.

Another um'd & ah'd but it seemed pretty obvious he didn't want to do anything beyond Option 1 either.

Any opinions? Would a warm roof be "doing it properly" or just a waste of money? Anything else I should be considering?