I must say that I actually think the location that was chosen for this project isn't ideal, even putting aside the SSSI-related hurdles that the Swanscombe Peninsula now entails.
I know that the site they've chosen is close to London, but from my perspective, one key pitfall it has is that it's far away from much of the rest of England by virtue of it being in Kent. Even within the London area, I reckon that they could have chosen a more ideal location.
For instance, I reckon that somewhere around the North West of London (along the lines of where London Heathrow Airport is located, and where Thorpe Park sits just outside the boundary of Greater London) would have been a stronger location. It would still have had the advantages of the London Resort site that was chosen in terms of proximity to Central London, but it would have also facilitated easier access from the rest of England; for instance, certain boroughs in the North West of London can be accessed in less than a 2 hour drive from Birmingham (the 2nd largest population centre in Britain, and the gateway to much of the Midlands). The chosen London Resort site takes 3 hours from Birmingham. Not to mention that London Heathrow Airport, the country's largest airport and one of the largest airports in the world, would have been located very nearby. This would have allowed for international visitors to fly in to a very heavily served London airport and access the resort easily, whereas the chosen London Resort site does not have that luxury; the drive time between Heathrow and the chosen LER site is over 1 hour. Even I, travelling from Gloucestershire, can reach Heathrow in under 2 hours...
Even still, if you look outside London, I personally feel that somewhere like Birmingham could have been a more ideal site for a UK theme park, particularly if catering primarily to the domestic crowd. Take a Birmingham borough like Solihull, for instance. Located in the South East of Birmingham, it's less than 2 hours from parts of North London and no more than 2.5 hours from many other parts of London, but it's also no more than 2 hours from many of the Northern population centres like Manchester and Liverpool, and it's very close to a population of ~3m (the Birmingham Metropolitan Area). A theme park site there would have had wide reach, being located less than 3 hours from most of England. It's not like it would have had no international visitation potential either, because Birmingham Airport, one of the country's largest airports, is located very close to that area. The M42 also runs very close, so motorway access is good. Correct me if I'm wrong here, but I seem to remember reading that a site in the South East of Birmingham, close to the NEC and Birmingham Airport, was strongly tipped to be the British nomination for a EuroDisney (now DLP) site, and I can certainly see why. To be honest, I'm quite surprised that the only theme park directly serving Birmingham is Drayton Manor... it seems like quite an ideal location for a UK theme park, from where I'm standing.
Back to the London Resort site, though; it's not even especially close to Central London compared to competitors. CWOA is no further away, and Thorpe isn't much further away. CWOA also has a key advantage of actually being within the boundary of Greater London while the intended London Resort site isn't, meaning that it is served by the London public transport network (I think?). This gives CWOA an advantage in easy travel to and from the park from within Central London (which I'm honestly surprised that Merlin doesn't exploit more, but that's besides the point), which the London Resort site would not have.
Come to think of it; why was the Swanscombe Peninsula actually chosen?