No. It's only valid as far as Northampton (or Long Buckby if you're including minor stations) on services from Euston or to Banbury if you're starting from Paddington (or otherwise using the route through Oxford).
https://www.network-railcard.co.uk/download/clientfiles/files/Map.pdf
It's a 'London and South-East Rail Services' map, and not just a 'Network Railcard' one. So it's only the cream-coloured areas of the map that fall within the Network Railcard area. (If you look along the edges of that area, you'll see the words 'Network Railcard Area' shown in red).