From what I gather, snapshots that could be used as a starting point for streaming updates have not been implemented yet. However you do have pages where I can download something like a snapshot for a couple of kinds of data:
https://download.companieshouse.gov.uk/en_output.html
http://download.companieshouse.gov.uk/en_pscdata.html
I can’t see timepoints in the returned data. Could you put the timepoint on those pages so we can grab the initial data and the timepoint to use for retrieving changes after that?