BBC link
If they need a height drop of 300 metres, it won't be in SE England at all, and most certainly not Essex. They might be able to get close if they went to the Surrey Alps or Berkshire, but one of the existing MTB sites in Wales would be much more suitable.
Or Fort William, which has already hosted the MTB World Cup six times, and the World Championships once. And coped fine with 500+ competitors and 10,000+ spectators, and got several awards for the best event of the series etc.
Unfortunately its rather unlikely to be there, as all of the Olympic venues seem to be decided purely on the distance from London (despite the claims that the Olympics will benefit the whole of the UK...). Fort William does have a direct train from London.
Though do they really mean "a height difference of at least 300 metres from the highest point of the course to the lowest"? That would require quite a bit of a hill. I assume they actually mean 300m of ascent / descent per lap, which is perhaps still a little on the high side - Fort William has 277m ascent per lap.