Commercial use vastly favours Mercedes. I'd wager a big chunk of the E, S and S-based Maybach models you see in Central are corporate transport, executive private hire or hospitality.
Keep in mind that using a private car in Central London could cost as much as buying an E-Class in parking fees on a monthly basis... An effect of this is that cars in use all day get seen more, even if there weren't actually more of them in the first place - which may explain why the regional difference seems so skewed.
Addison Lee, who I think are one of the largest private hire companies in London, claimed 10,000,000 annual customers in London, and London's largest fleet of Mercedes cars, their fleet being in excess of 4,500 cars in London alone. They're also far from being the only player in the market, but even then, I've never had a ride in an executive taxi that wasn't a Mercedes. The last Maybach I saw in London was private hire (£170/hour), operated by London Luxury, whose offer E, S, V and Maybach from Mercedes, but only i7 from BMW.
I mean, most of the 7-ers you see on London are probably on similar duty, they're just not as popular for the purpose.