During the relatively severe winter of 2010, many local authorities ran short of salt (and, in addition, the sustained low ambient temperatures exacerbated the 'lack of salt' problem), leading some local authorities to experiment with additives such as molasses and other alternatives to the 'usual' rock salt (which was coarse pink halite - now 'we' use finer white salt).
The following outlines the emergency reserves held by the government (local authorities are expected to hold their own stocks locally):-
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/571267/strategic-salt-protocol-note-2016.pdf