Ronan I wouldn't bother with the foil against cavity blocks but would be better than nothing. The main benefit you would see is the air space between the cavity block and suggested plywood however small.
I have had bubble foil over steel doors for 4 yrs exposed to elements and visible internally, but getting it to stay put is something else. It works good and reflects internal heat. However you would get better effect just by layering plywood with an air space between. Mines lined with MDF interlocking floorboard think around 8ft tall painted over, must have done it 16 years ago. Just an air space created which will be better than plywood, at least you can screw in to it rather than plywood then in to the battons. Downside the MDFchipboard is 18 1/2 mm thick but come in 22 1/2" wide interlocked + air space/battons.
Roofs just 1/8" ply done 22 years ago, massive improvement. Recent times cheap insulation in between the rafters gave an improvement. Its now topped off with mdf which has given absolute minimal improvement.
One of the biggest improvements is laying something however thick on the cold floor. Two years put down that cheap ribbed stuff, not only does it warm up quicker it stays warmer for longer, not to mention dropsies and damage.