I was thinking - God I should know this ...
if you go here
https://eservices.landregistry.gov.uk/www/wps/QDMPS-Portlet/resources/example_register.pdf
you will see that there is a space for mortgages
This is because clearly people need to know if there is any bar to selling ( like the mortgage company has to be involved ) - an entry can certainnly be entered by the mortgage company
but I imagine the answer you want is whether the mortgage MUST be entered on the land registry and I think the answer is no
( that is if there is no mortgage recorded the the co doesnt get its money and I am sure that is NOT the case )