I have an idea for an alternate history world that deviated from ours in 1889 when something wiped most of the human race outside Britain.
I want to know, in that time, how badly it would effect britian's food supply to suddenly be cut off from international trade.
This source:
http://www.ifr.ac.uk/waste/Reports/RuSource_foodsecurity.pdf
Reports that Britain was 60% self sufficient in food in the 1870s and only 40% self sufficient in 1914.
That indicates that in 1890 Britain's food supply would be badly impacted if trade were cut off.
I imagine that in your imaginary scenario, this would add impetus and accelerate the temptation to rapidly colonize areas outside Britain. France, Belgium, the Netherlands (and Ireland, if your "something" only spared the island of Britain, rather than the British Isles) all have rich agricultural lands nearby. Rapid colonization of their now depopulated farms would alleviate Britain's food crisis.
You might find this article interesting,
Paul Clayton and Judith Rowebottom, 'How the Mid-Victorians Worked, Ate and Died' in The international journal of environmental research and public health 2009. @ http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2672390/#__ffn_sectitle