Well, it happens that I work on railways so I know a bit about drainage as well.
Pffft, I bet all you know is ballast. :) #railwaystereotypes
And you are right: for the relatively small investment, a lot can be improved. Drainage is "not sexy", so these projects don't get too much attention. Mostly it is reactive and only until something like that happens, it gets attention required, but then: most of the budgets are thrown on the responsive measures.
Yes, it's exactly the same in road infrastructure, which is what I know about. If you don't look forward (quite a way), you end up getting nowhere/firefighting all the time. Also, most specs that have been followed in highway construction for decades are now well outdated--just look at the A406, again.
Having said that, it is worth to weight it if the money spent to mitigate 1 in 100 years event is worth it and I personally think that I am happy to give up the right to use underpass once in a long while if it means the money is spent elsewhere more efficiently.
It's not by any stretch of the imagination a 1-in-a-100-years event. These will become more frequent. We'll be here again in only a few years' time. Also, quite simply, you build the best drainage at the lowest point, and that'll help you in all sorts of other events, too, especially if the infrastructure at higher points is badly-maintained. My point wasn't about 'giving up the right' to use the underpass, but about the very real risk bad drainage there represents. Apart from the issues of personal safety, as you're a railway person I don't need to explain the risk to railway bridge foundations from standing water. Many railway bridges in London are Victorian, so well overspecced, and I don't know if they were ever built with the modern idea of a 'design life' in mind, but there's still no need to risk deterioration that can be prevented for a much larger range of events. (I don't know from when the bridge at Worcester Park is.)
But most of those points are engineers' points, that is, people who operate very much within the system and have no leverage over it, having to make these sorts of 'triage' decisions, and I completely understand you making them, but there is no need to transfer that perspective to the political sphere.
I'm mainly pressing the point that we must not accept inaction on things like this and that the parameters under which engineering operates can and should be changed. Just witness the ridiculous row and political point-scoring about Hammersmith Bridge in the last couple of years, led by an evidently incompetent Transport Secretary. I've heard so many excuses over the years for why this, that, or the other can't be done, and seen ridiculous, frivolous allocation of funding that has completely ignored actual priorities, when all it needs is political will and it can and will be sorted out. Granted, I'd agree that this isn't the absolute number one priority in highway engineering in London, but it has happened so often in the last few years, and not only in Wallington, that it simply has to be addressed. And for that, there is a need for public investment that the usual suspects can't dodge or shirk.