Narrow island platforms

mikebuzz has raised a very interesting point (or, at least, I think so). The CSLR seems to be the only deep-level tube to have gone in for underground island platforms. I say underground, because if you look at the (then) Bakerloo extension to Stanmore, most of the stations in the open air past Finchley Road were island platforms, and yet Swiss Cottage and St. John's Wood weren't. In fact, the platforms on these two stations (like at Highgate, which is roughly the same vintage) are quite far apart so that the escalators could come down between them. By the 1930s, it seems the Underground could be built without the wayleave restriction of routing them under streets as had been the case twenty-five years beforehand (does anybody know exactly when this rule was relaxed, by the way? Was an Act of Parliament involved, or was it just custom and practice?). In any case, north of Finchley Road was really just a re-arrangement of four existing Metropolitan tracks andt it must have just seemed easier to put one platform between what became the up and down Bakerloos.
But the CSLR remains a bit of a mystery. If you take King William Street out of the equation (because it was a special case), I have a theory that works except for one station. The original Parliamentary Bill specified that both the lines would be in twin tunnels, but it didn't specify that all the stations had to be. Borough, Elephant & Castle, Kennington and Oval were all built with separate platforms. However, the original terminus, Stockwell was an island platform, which - if you look at the original Engineering diagrams - just fitted inside the road at the point. Indeed, the twin tunnels broke into one single tunnel just before the spur to the Depot at Stockwell and the scissors crossover just north of the site of the original station. So maybe here's a good reason why Stockwell was an island platform: it was a terminus with a scissors crossover attached to it. Ten years later, two extensions pushed the line in 1900 both north and south: to London Bridge, Bank and Moorgate which were all single platforms, and Clapham North and Clapham Common which were both island platforms. Like Stockwell, there is a sort of logic in that Clapham Common was a terminus with a scissors crossover within the single tunnel. OK, this logic doesn't quite work for the intermediate station, Clapham North, and that remains my only exception to the rule. To the north, at the time of completing Moorgate station, plans were already afoot to extend to Angel, so it was only ever going to be a temporary terminus and thus there were single crossovers both to the north and the south of the station. Neither Old Street nor City Road were ever intended to be termini either and the platforms there were in single tunnels. Angel was an odd place to site the terminus of the line, but the Metropolitan Railway objected to the CSLR reaching King's Cross on the grounds it might interfere with Moorgate-King's Cross traffic and so it stayed a terminus for six years and was constructed as an island platform. It's worth noting that by 1901, the Central London Railway had opened, and even though it had two underground termini at the time of opening, Shepherd's Bush and Bank, none of the stations on the line were island platforms. And although the three tubes that became the Yerkes tubes were still at the planning stage, none of them had any island platforms. When the final extension of the CSLR came in 1907, King's Cross was in separate tunnels, while the terminus at Euston was an island platform. If I'm right, both the CCE&HR and the CSLR opened in the same year and both were constructed under Euston station itself. This all leads me to believe that limitations of space above the line was not the deciding factor in the construction of CSLR Island platforms.