It doesn't always have to be a populated valley that gets dammed. There's a decent selection of disused quarries, maybe some of the Highland lochs that have potential to create something like Dinorwig at minimal disruption to environment once complete. What I don't know is how those constructions compare in today's money to alternatives. Still, £70bn brings an awful lot of choices, especially when lifespan is indefinite, just periodic turbine replacements.

If the geography isn't available there's the Netherlands project that's pumped storage on entirely flat land. Involves building an artificial lake, with a cavern and turbines below it. Suitable rock for the cavern is quite a long way down, which adds to the cost, but also adds to the head so it should end up with significant capacity. As far as I know it is the first such scheme, so costs are projection only at this stage, but it looks pretty interesting if it turns out to be viable economically: https://o-pac.nl

Or the UK company that has a plan involving winching weights (thousands of tonnes) up and down disused mine shafts. The open question of course is how many of the world's thousands of disused mine shafts are going to be viable. If viable the advantage seems to be very low surface profile.