I can think of a few other candidates locally. Much of London's expansion in that era revolved around the expansion of the London Underground network- that's what led to the eventual abolition and absorption of Middlesex County, after all. And the London Underground formerly extended to
Windsor & Eton Central Station, in Berkshire- with this extension of the Metropolitan District Railway (now the District line) launched in February 1883, with ten trains a day running from Mansion House, via Castle Hill (known today as West Ealing station), Hanwell, Southall, Hayes & Harlington, West Drayton, Langley and Slough, before terminating at Windsor & Eton.
IOTL, this service was discontinued, having proved 'uneconomic', after 30 September 1885. But given its status and location, serving both the permanent residence of the Royal Family and the most famous, arguably most prestigious school in the country, one could easily imagine the Old Etonians pulling strings to keep the service running, even at a loss. And if it had kept running, especially after the creation and expansion of the Slough Trading Estate, one could easily imagine a slightly expanded Municipal Borough of Slough- perhaps including much of SE Buckinghamshire (at least as far north as Denham, which was originally planned to have been the terminus of the Central line's westward extension under the London Passenger Transport Board's 1935-40 New Works Programme, with work on the extension postponed due to the outbreak of World War II, then cut back to West Ruislip due to post-war austerity)- along with Windsor (which would've almost certainly have included Eton and much of the rest of the present-day Parliamentary constituency, and been a separate borough, locking it in as the most solidly Conservative London Borough)- being the westernmost parts of the Greater London area. What do you reckon?