We did indeed have a drought in 2011, and there were no ill-effects on this system. That's the point. Water shortages in the UK tend to affect the southern end,

We did indeed have a drought in 2011, and there were no ill-effects on this system. That's the point. Water shortages in the UK tend to affect the southern end, and most of these hydro buffers are up north, where they wish it would stop raining. Increasing loads on the system won't really effect this type of setup, hydro is used as a peak buffer not for base load, peaks like these will tend to have the same order of magnitude (they have over the past 40 years while base load has increased). There are other issues with power in the UK, but dealing with post TV tea peaks isn't one of them, and isn't worth investment which could be spent better elsewhere on the grid.