This is a list of some of the most promising sites in Scotland, plus a few in England and Wales. There are more, particularly in Scotland where many of the unlisted ones have greater potential than any of the English or Welsh ones. I only give areas at the surface and at up to three depths for each site. The GWh figures are based on the assumption that the air vessels are at the given depths, and are of such a size that filling and emptying them causes the water level in the lake to rise and fall by no more than a metre (except Loch Lomond, limited to about 20cm for practical reasons). There’s no magic about the particular depths: you could have multiple systems at different depths in the same lake, possibly one above the other in some parts of the lake, and you could achieve the same energy storage with less variation in water levels by doing so.
Those assumptions are very arbitrary. You could store (much) more energy, at the cost of causing water levels in the lakes to rise and fall more as the air reservoirs fill and empty; or you might choose to affect the environment less and accept a smaller capacity. If the engineers of conventional pumped hydro systems are honest, their figures are of the same sort: how big a dam do you want to put across that high valley for your upper reservoir? How much do you want the water level in your lower reservoir to vary? (One assumes that they’re happy to allow the upper reservoir to fill and empty almost completely.)
(That particular point has an interesting consequence for underwater compressed air in the lower reservoir – you can pump water from the lower reservoir to the upper at the same time as filling the air vessels under the lower reservoir, which reduces any change in level in the lower reservoir, possibly to zero. This is already relevant at Loch Ness and Loch Awe, and could be at many future sites.)
(Areas are in hectares, depths in metres. Map numbers are OS Landranger series, which have the underwater contours that I used to find the approximate areas at various depths. The GWh values are the actual energy values of the compressed air: it would take rather more energy to compress it, and you would get rather less back – but the values are only approximate anyway.)