The 300 Mb areas on Codelook are probably Virgin Media coverage. The data from Mr Max will not be related - note how old the BT Wholesale data set is.
Magenta Telecom SQL Database, Version 1.3
Creation Date: 5th February 2020
Source: Ofcom CodeList numbering database dated 5th February 2020
Source: BT Electronic Price List dated 5th February 2020
Source: BT Wholesale broadband datasets dated 15th August 2019