How can I insulate the floor of a room above a porch?
Actually, since you are primarily concerned with heating conditions, the vapour barrier goes on the warm side of insulation, meaning right up against the sub floor boards. This is to keep moisture from the house from condensing in the cooler insulation. Outside moisture would not condense in the insulation as it should always be warmer than the dew point of the outside air, though in the UK it may often be a close call.

Loose fill generally provides a more complete insulation job, filling the whole space completely. Batt insulation is much easier for DIY work. Even if you access from below, loose fill may be possible. You could actually install loose fill by cutting a hole in each joist bay, instead of removing the ceiling. The main problem is you have no way to verify the void was completely filled. It is highly dependent on the skill of the installer. It also negates the ability to place a vapour barrier, but depending on the flooring system, it may not be a serious issue. In your case though, it is probably an issue.

If any recessed lights are in this space, if they are not rated for placement in insulation, they will need to be replaced. If you go in from the ceiling, it will be hard to get a good vapour barrier. Just do the best you can, sealing it well to each joist. You'll have to allow the joists to be part of the barrier. It may be worth spraying them with a quick latex primer coat to help make a better barrier.