Whipsnade's visitor base is predominantly ZSL members and recurring visitors from the surrounding area. What also works against it is it's location-it's not an all-year/all-weather zoo, unlike London.
Significant strides are being made to rectify these issues...
I wonder how to certify Whipsnade location

The only solution would be to develop a road and train line specially for visitors. This would be possible in many cities which purposefully develop tourist potential of their attractions, but seems somehow unachievable in London.
"Solving issues of distance and not all-year" would be precisely a new zoo somehow intermediate to London Zoo and Whipsnade in character. Closer with easy access by city transport, with all-year animal houses but with enough space to show big mammals and large herds.
I think most people agree that there is big public interest to see diverse collection of large animals in London.
And most people agree that safari park outside the city doesn't fill this demand. Also, this role is not filled by places not presenting big animals: aquariums, sealifes, petting zoos and small animals like biotas etc.
I would argue that a third zoo is better that trying fight unsolvable issues of space in London Zoo or distance to Whipsnade. Successful example of a crowded city which has good zoos is Singapore, which is a city much more modern and froward looking than London. They have 3 zoos literary at the same public square: Zoo, River Safari and Night Safari. City of Singapore did not seek to extend the Zoo into extra space, which would be the only way a Londoner can think about things. It developed three separate attractions each with different flavor. It doesn't mind that many animal species overlap. And can charge 3 times for tickets. Singapore also has 2 aquariums, a bird park and perhaps some smaller animal attractions, which are again not seen as a competition to zoos.
I think zoos preserve and extend the value of green spaces in the city, rather than 'destroy few existing green spaces'. Zoo is not factory halls! Zoos certainly increase recreational potential of urban green spaces. They also preserve most of the area vegetated and not developed, and typically extend it by planting trees and other vegetation. London Zoo is here especially good example, for it has far more trees and biodiversity than grass lawns of the surrounding park.