I've used Addressbase a LOT before and they're separating out the core[0] of it to give it a more permissive license[1]. It has the Classification Code attribute which has an identifier for the kind of property you're looking for[2].

Not sure on the licensing details for this but I think your use could be accepted. This could work as a more established alternative from the true golden source of property records in the UK - if you ever fancy switching over.

[0]: https://www.ordnancesurvey.co.uk/business-government/product...

[1]: https://www.gov.uk/guidance/access-free-address-data-using-a...

[2]: https://www.ordnancesurvey.co.uk/business-government/tools-s...