The land registry title number is unique to your property. You should be able to find it if you look at files related to your house purchase. Perhaps a final letter or email from your solicitor when your house purchase went through. It’s an essential piece that is needed for the form.
If you can’t locate your title number you can pay a small fee (£3) to order it from the Government website: https://www.gov.uk/get-information-about-property-and-land/copies-of-deeds