Seriously, if you are going to do it, then i would look at somewhere closer to the Chiltern Hills... Like the places i mentioned previously... In reality, there is some fun, fast riding round where I live... I can't really say much about where Herts shore is, but i believe it's not as hilly. As for epping forest, i think there are places as close to london more or less, with more variety... if you head over wendover direction, the hills might not be too bg, but there are lots of them... You're legs will definitely feel it if you try hard enough... And you can also descend for a good 3 or 4 minutes solid!!
I'm not trying to annoy anyone else by saying that, i just think that the chilterns would be your best bet...Plus