Claire - the restriction is known as a form A/joint ownership restriction and most commonly associated with the term 'tenants in common' and a severance of joint tenancy -
Joint property ownership: Overview - GOV.UK
I suspect, but hopefully your parents can confirm, that they may have written/amended their wills and/or taken advice re joint ownership/inheritance after they had purchased. That advice might have included applying for the restriction you refer to
LA407933