You’ll find that you might find opportunities, but you really need to be buddies with a good agent.
Most owners in an old building could be anywhere. Some might be elderly, some might have emigrated to UK,, Europe, USA etc, hence the difficulty in finding 80% of owners.
Companies such as Richfield and Mainland property development companies have been involved in many redevelopment projects and they are always targeting prospective buildings and landlords. They are also very close with all the real estate agents, hence they are likely to get first dabs on any deal, way before you hear about it and they can act very quickly.. Hence, in reality there are very few opportunities.