British writers who use some time in those instances where Americans would use the closed compound are following the advice of Henry Watson Fowler (d. 1933), whose preference for restricting sometime to adjectival use has been repeated in subsequent editions of his 1926 Dictionary of Modern English Usage:
Some time, sometime, etc. advv. Some time is often used elliptically for at one time or another. There is no essential objection to writing it some-time or sometime, but it is convenient to keep it separate words for distinction from the sometime that appears in such descriptions as ‘sometime Fellow of …’, ‘sometime Rector of this Parish’, meaning formerly. — Henry Watson Fowler, A Dictionary of Modern English Usage, 1994, 550.
Fowler juxtaposes “no essential objection” to “convenience”: he does not declare the adverbial use of the closed compound an error, merely a way of keeping adverb and adjective neatly apart.
Many writers on the BBC website follow Fowler’s preference:
Salford City Council said the theft, which happened some time last week, had been reported to police. — BBC News/Manchester, 20 Aug. 2018.
Caernarfon Crown Court heard the filming happened in Liverpool some time between January 2007 and December 2009, and it was found by an ex-partner. — BBC News/Wales, 9 Aug. 2018
The alleged “assault or assaults” happened some time before 03:45 GMT on 11 February, the court heard previously. — BBC News/Hampshire, 11 Dec. 2018.
Network Rail says the fire damage was so extensive it has had to rebuild the equipment room from the ground up but hopes to have all lines back open "some time Friday morning". — BBC Newsround, 21 Dec. 2012.
On the same BBC website, however, sometime also appears, especially in news sourced from Northern Ireland and occasionally from Wales and Scotland:
The Northern Ireland Environment Agency (NIEA) found that the spillage had happened sometime in the late afternoon on Friday 10 June.
Police said the incident happened sometime between 21:30 BST and 22:00 BST near a local school. — BBC News/Northern Ireland, Foyle & West, 26 Oct. 2018.
The crash, which happened sometime before 01:30 GMT, involved a blue Vauxhall Astra. — BBC News/Wales, 21 Nov. 2018.
About 900 litres of syrup leaked on to a road in a Highland village after the container holding the sticky liquid was vandalised. Police said the vandalism in Conon Bridge happened sometime between 16:00 and 18:00 on Sunday — BBC News/Scotland: Highlands, Islands, 21 Sept. 2015.
The National Audit Office is already investigating the economic case for the roll-out of smart meters, and is due to report sometime this summer. — BBCNews/Business, 21 July 2018.
The last chick emerged from its shell sometime on Saturday morning to join its three siblings that hatched on Friday. — BBC News/Norfolk, 3 May 2016
Greater Manchester Police said it was investigating after being called at 10:15 GMT on Monday to the building in Manchester Street following reports of a burglary, which is thought to have happened sometime after midday on Saturday. — BBC News/Manchester, 9 Jan. 2019.
The vandalism at St Mary's Church in Mendlesham, near Stowmarket, happened sometime between Friday evening and Saturday morning, Suffolk Police. — BBC News/Suffolk, 10 Jan. 2018.
Though there may be an English preference for the open compound, the presence of both forms on the BBC website suggests the choice of open or closed is left to each writer rather than being proscribed, say, by a network-wide stylesheet. Given Fowler’s “no essential objection,” it’s not surprising that some writers fail to see a “convenience” in an orthographical distinction for a fairly infrequent adjective.
In the United States, however, the closed compound is the overwhelming choice:
They say the incident happened sometime last week. The group believes the vandal or vandals were trying to break into the church, but were unsuccessful. — 6-ABC, Philadelphia, 13 July 2018.
Daphne Kirksey, spokeswoman for Tennessee American, said the break happened sometime between 5:15 and 5:30 a.m., allowing water to spill downhill to Scenic Highway with bitter cold temps turning some of it to ice. — Times Free Press (Chattanooga TN), 31 Jan. 2019.
The shooting happened sometime before 3 a.m. at Ben E. Keith Foods warehouse in the 700 block of Cravens, according to the Missouri City Police Department. — Houston Business Journal, 20 Aug. 2018.
Although they would be considered nonstandard, there are scattered instances of the open compound:
Sheriff Moon said it happened some time between 7 and 8 a.m. Monday near the 1900 block of Skyball Road. — 6-WBRC, Birmingham AL, 15 Jan. 2019.
She explained another incident happened some time between Monday afternoon and Tuesday afternoon, November 12-13, … — South Pittsburgh Reporter (PA), 20 Nov. 2018.
The phrase some time before x occurred far more frequently than with other prepositions:
A 911 dispatcher said the explosion happened some time before noon. — TribLive (Tribune-Review, Western PA), 31 Oct. 2018.
It happened some time before 11:45 p.m. Thursday in the area of North Mollison and Park avenues, near a back parking lot to the high school, according to a statement by the El Cajon Police Department. — KUSI, San Diego, 22 June 2018.
Although it can reveal broad differences in usage, the designation in Google Books as British or American is wildly inaccurate. Indian and Australian sources, for instance, can wind up anywhere, not to mention scientific literature written by non-native speakers.