The Uxbridge Loop was proposed in the last years of the nineteenth century, but until 1899 it was proposed to run further west and link Denham with West Drayton. Work on the section at the north end started in 1900, but only opened in 1907 after the main line. By this time the Metropolitan were operating to Uxbridge via Ickenham and the GWR seems to have lost interest. Only a small amount of work immediately south of High Street station was ever carried out. The east to south curve at the junction, necessary for the circular service, was probably never used and was lifted in 1916. The whole line was closed in 1915 and when reopened in 1920 it had been singled. The line closed to passengers in 1939 but a sparse goods service to the siding that had been belatedly provided at Uxbridge continued to 1964 and to a fuel depot at the south end of the east to south of the curve for another year or so.
Sources: G.W.R Disused Stations in Greater London (Connor, Connor & Butler, 2000) and Section 26, Second Edition, of Cooke’s Diagrams.
One point worth mentioning is that all lines between Harrow South Junction and
Aylesbury South Junction, previously Met & GC Joint, were transferred
to LTE on 25 June 1950; LTE lost all lines further north. At 00.01 on 11 September 1961 the lines from just over one-and-a-quarter miles north of Amersham were transferred to the LMR.
I don't have a Newspaper Archive subscription so I can't be certain, but from the
excerpts I can read, it looks like the second island platform at Moor Park at least opened on 23 April 1961. Another
article of 29 December that year said that Moor Park to Watford South Junction hadn't yet been completed.
It seems that a platform was built on the new pair of lines and traffic transferred to it, after which the original ones were closed and a new platform built. Only from 23 April 1961 were all four lines and both platforms at Moor Park in service.
One idea current after Beeching, although it wasn’t in the report, was to close Amersham-Aylesbury; Aylesbury would probably still be served from Paddington via High Wycombe. As an alternative to buses LT considered electrifying to Great Missenden or Wendover, although the latter was considered likely to not be worth it for the amount of traffic. However, rising traffic and expected capacity problems knocked this on the head, for a while at least…
Source: The Metropolitan Line (Horne, Capital, 2003).
EDIT: underlined sections corrected.