Marc - we have had requests from individuals seeking to provide evidence that they do not own property in the United Kingdom. HM Land Registry deals only with property in England and Wales. Scotland and Northern Ireland have their own registries.
Only 80-85% of the properties in England and Wales are registered; therefore it is possible to own property/land in England and Wales without HM Land Registry being aware of it. No other body holds details of property ownership of unregistered land. There is no single body in the UK which is able to supply such information.
It may be possible to undertake a PN1 Search. This will give details of any registered property held in England and Wales by a named individual either in their sole name or as a joint proprietor. This may throw up false positives as the search is against a name only and not an identifiable individual. For example, a search against John Smith will throw up hundreds, if not thousands of results, none of which may be related to the actual person making the search. There are restrictions as to who can make this search. Please see Practice guide 74: searches of the index of proprietors' names for guidance.
Please feel free to forward this information to whomever it may concern, namely those who are asking you to prove such a negative.