OS has advised  that it is currently exploring the concept of ‘perpetual pricing’ for its AddressBase products https://www.ordnancesurvey.co.uk/business-and-government/products/addressbase-products.html with the aim of introducing consistency with Royal Mail PAF Cleansed Data terms.     If this is of interest to you please feel free to contact Santiago Jagot at Ordnance Survey @ santi.jagot@os.co.uk by 13th October 2018