Not a Brit but, if I were to guess, there are probably (relatively) few water reservoirs around precisely because of the weather. And, if someone were foolish enough to try and build one, they'd be buried under mountains of red tape; we've seen the years-long journey it took just to get a permit to lay down some gravel and build a goddamn parking lot. Carving out a new lake or dam would whip bureaucrats into a frenzy of demanding theses on environmental impacts, historical preservation, local safety, preserving natural beauty, preserving local character/aesthetics and who knows what else.
Speaking as a Brit..... the weather isn't really the issue (as I type this under a heavily overcast sky with the sun a distant memory and constant rain forecast all day in mid June).
Although there has been some building in the last few years, reservoir capacity remained static from about the 1970s to a few years ago despite significantly increased population and water demand. The issue was exacerbated by shocking levels of leakage from supply pipes that was also largely ignored. As the country stumbles from financial crises to bankruptcy to financial crises it was an easy way for politicians and officials to reduce significant capital expenditure without immediate short term consequences (i.e. when the shit hits the fan those responsible have long since taken the money and ran without accountability). It's also not "fashionable" expenditure. Can't spend money on basic utilities when it could instead be spent on narrowing roads for bus lanes and cycle tracks or painting rainbows on crossings. There's a similar issue with water drainage. Why maintain or, heaven forfend, dredge when you can do nothing and roll out the shocked Pickachu face reciting "no-one could have seen that coming" as people are flooded out of their homes.
The regulation (esp environmental) is a significant issue but not the fundamental one. When the Government wants an infrastructure project to happen it happens. It used to be by private Act of Parliament granting all necessary rights (and compulsory purchase powers). That's how the great railway expansions through the 1800s to the early 1900s happened. Now we have the Transport and Works Act 1982 which is an administratively simpler method. Work up the scheme, advertise, invite representations (objections), hear the objections at public inquiry following which if the scheme wasn't a basket case from the start it gets every permission, consent, approval required and compulsory purchase powers. Private Acts can still be used and are for properly big stuff (Crossrail, HS2, Docklands Light Railway) sometimes in conjunction with the TaWA (hybrid schemes).
The thing to remember with the environmental regulation (mostly European) is that very little of it actually prohibits consents being granted. The strongest relate to birds and habitats of flora/fauna but even those have get outs (usually). The generally applicable environmental assessment stuff (built into the planning permission system primarily) does say permission can't be granted without taking account of significant environmental effects (and saying you've done so on the piece of paper giving permission) but not taking account of a significant environmental effect would have made a grant of planning permission unlawful regardless. The basic rule of administrative decision making is; take account of all material consideration, disregard all immaterial considerations and, don't decide what no rational decision maker could have decided (capricious, perverse, arbitrary) and it's been that way well before environmentalism. A significant environmental effect of a scheme will always be a consideration material to the determination of permission for that scheme.
Environmental regulation has made things more procedurally difficult and until recently the UK Courts said non compliance with a procedural requirement of EU derived rules meant the decision gets quashed regardless of whether the non compliance made any difference to the outcome. A decade or so ago, it was before Brexit, the Courts reversed that position but it's never good for a scheme promoter to be in Court arguing that some non compliance was irrelevant to the outcome.
The real problem with the environmental regulation is it gives mechanisms to those who oppose the scheme regardless of whether they give a damn about theoretical impact upon some slugs to impose delays, costs and political pressure and the last of those three can be the most significant because politicians tend to be cowardly and maliciously self serving scum whose concept of "the public interest" is derived from whoever is most loudly and currently shouting at them.
And that was Clarkson's problem. He needed permissions from politicians (and officials answerable to those politicians) who firstly, saw more to gain in appeasing loud local NIMBYs (Not In My Back Yard) and secondly, take malicious delight in fucking over the bad man they don't like. (He also made some dumb decisions - maybe for dramatic TV reasons, who knows.)
