I have a Merc. It is exactly as a Brompton but cheaper. You can buy it on Ebay or check Mercbike.com. I've bought the bike for the same reason as you.
At the moment I am travelling on the underground in my way to work and riding in my way home. I am doing 12 miles to home but soon I will start commuting by bike both ways with a total of 24 miles.
My experience with the Merc is just wonderfull, it costs £330 and comes with absolutly everything, including in the price the front bag and the big bag for the bike, etc,etc. With the Brompton you have to buy all the accesories separated.
The Merc is very well build. I don't want to start a discussion about Brompton/Merc, there are other threads about it. I am just giving my opinion as a owner of one. I am MORE than happy with it.
I have the bike with me wherever I go and I have never had a problem in the underground in rush hour. I am using the Districtic Line and/or Hammersmith and City Line.
Hope this helps.