Essex, highest point 147m, is not a county noted for its mountains. It was therefore perhaps no surprise that inspectors from the International Cycling Union deemed the county’s Weald Country Park just too flat to host the 2012 Olympic mountain bike competition. Any proposed course would not challenge the riders sufficiently, they said.

...the UCI’s Mountain Bike Sports Co-ordinator, Peter van den Abeele, said: “You don’t necessarily need to have real mountains to stage this event, it all depends on the course. The most important thing is to find a suitable venue and we will find one: that could be one and a half or three hours' drive from London, but who says we cannot find a venue in Essex?”

The options within the county are limited, however, with Epping Forest being seen as the only viable alternative to Weald Park. Mr van den Abeele said he would be returning to England in the next few weeks to look at other possible sites for the event, but remained tight-lipped on where they might be.

From: UCI reject Essex's Olympic mountain bike venue, Mark Appleton, Bike Radar, 13 Feb 2008

More at: Bike Radar

LOCOG had opted for the 500-acre Weald Park partly because the lakes and tree-lined avenues would provide nice pictures for television and is easily reached by the A12 and M25 with rail access from central London.
Ray Green

Epping Forest attracts large numbers of mountain bikers. Mountain biking is generally permitted except around the Iron Age camps, Loughton Brook and other ecologically or geomorphologically sensitive areas. Despite clear signposting, a minority of mountain bikers continue to cause damage in these areas, and the Conservators of Epping Forest have expressed their concern. A number of clubs organise rides, particularly on Sunday mornings.

Epping Forest was considered as a venue for the mountain-biking event of the 2012 Summer Olympics, though the final choice was Weald Country Park near Brentwood, Essex.

Source: Wikipedia

See also: Epping Forest Mountain Bike Riders

I don't doubt that if the Epping Forest is chosen as the location for this event the promo hype will be full of reader friendly concepts like 'minimum impact', 'environmentally sensitive', 'sustainable', 'legacy benefits'.

Knowing the LOCOG/ODA from the receiving end as we do, the promises will sound great but the outcomes will be shoddy, overpriced and will have seriously adverse impacts on local people and their amenities. I fear for the fragile eco-system of the forest and the loss of natural amenity during the construction process and the events. Not to mention the cost and disruption of demolishing the site afterwards.
Martin Slavin

It'd need an act of Parliament or something to get it at Epping Forest - as organised sporting events are not allowed there as it's Corporation Of London-owned. And, of course, its highest point is a mere 93 metres, so how they'd get a 300m difference in is beyond me.
Comment from Bike Radar by PSIMONK