I've just been reading the document set out by the gov't

and it says this 

"SIS II is used by EU and non-EU Schengen members (Switzerland, Norway, Icelandand Liechtenstein). The UK will continue to use SIS II until the end of 2020."

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/868874/The_Future_Relationship_with_the_EU.pdf Is the full document if anyone wants to read.