What you are probably referring to is this website.
https://eservices.lan...jPWDU_P0C7IdFQG9k5Tz/
You can use it to check whether LR finds a record of your property (if it does then it is definitely registered) but you don't have to buy it. It doesn't tell you the LR number until after you download the record.
However if you don't find it using the search, it doesn't necessarily mean the property isn't registered, just that the search can't find it. This has happened to me. It is fine if you happen to live at 49 Acacia Ave, Snodgrass-on-Sea, but rural properties without a conventional address it seems to sometimes miss.
If you do the search, can't find it and yet the date you moved in is later than the date of compulsory registration of your district (see the link I gave you to find out), it is worth phoning the LR office that covers the area where you live to ask - they are quite helpful with laypeople.