Ooh! (Transportation and urban activist here.)
So, first, Britain doesn't have that many metro areas relative to its size, at least by comparison with France. Look at how many million+ metro areas Britain has and how many France has. Also... the French banlieus are horrible places.
Second, if you want Britain to have less sprawl, then you need to knock the political causes of sprawl: namely, the garden city movement, which itself came out of the general hatred the urban elites had of the living environment of the urban working class. In the US, this hatred had an ethnic component (WASPs hating on white ethnics), but in Britain, it was purely about class. More fundamentally, because Britain industrialized first, it had a huge city and several big ones before modern advances in public health, and this colored people's perception of cities. Paris urbanized early, too, but was also Hausmannized early; New York urbanized early and had the exact same anti-urban elitism as London.
An earlier socialist movement would have probably done the trick: postwar Labour engaged in a lot of urban renewal, but late-19c socialists focused on higher wages and shorter working hours, and had living conditions on the East End improved earlier, there would have been less impetus to build modernist housing estates.
Alternatively, a less functional government would also have done the trick. In New York, the reason the areas developed by the subway have red-brick apartment buildings and not modernist housing estates or single-family houses is that the city was stuck in fights between the ethnic machine and the WASP reformers, so by the time it managed to get the subway built, there was so much pent-up demand for housing that the new housing was dense, contrary to the reformers' hopes of building single-family houses to properly Americanize all the immigrants. (References: the
Historic American Engineering Record.) Conversely, in Sydney, early development of commuter rail is cited as one reason the city has low density: it made it easier for workers to suburbanize early. So your POD could involve the London Underground taking much longer to build, to the point that Metro-land would be much denser.
In the postwar era, you'd want to find reasons for the state to build fewer roads and more trains. Maybe the postwar governments decided to follow Japan's example and invest heavily in public transit while limiting motorization, in order to reduce oil and car imports. It wouldn't give London a Parisian density, but it would extend OTL's Outer London density farther into the suburbs.