I'm no expert, but I was reading this earlier and it appears you can (as long as you leave a 50mm air gap)
http://www.celotex.co.uk/downloads/Between%20and%20under%20rafters.pdf
Unfortunately the joists aren't deep enough for insulation, which is why I was going for under rafter Celotex.
I don't think moisture build up will be an issue between bubble wrap (under joists) and outside tiles as that space is well ventilated (there is no felt or membrane under the tiles...
Our loft space has tiles on battens with no felt/membrane underneath.
The underside of the rafters (which are not deep enough for between-rafter insulation) have a layer of foil/bubble-wrap stapled to them to keep dust/wind out and provide a bit of insulation.
I was thinking about adding...