cheapest way to get from braintree to victoria (to connect with the cheap fare shown above) likely to be by olympian coaches service 743, only 1 service per day dep braintree 06h10, return from victoria 17h10, journey just over 2 hours. (their site is down, cannot determine fare.)
next cheapest by N-Ex from Chelmsford, �11-30 return but leaves you to make your own way to chelmsford by local bus.
train braintree to london zone 1 is �27-00 return.
btw, by train to dover (braintree-stratford-waterloo-dover) is �39-00 return and would be about 90-120mn quicker than by coach.