What can I say about Swinley - if you are in London, get on out there and ride. There are jumps, obstacles, varied trails, and tons of sweet singletrack. If you can find a local or two to help you find your way around, then you'll be in great shape - otherwise you can buy a map in the Outlook. You can rent (hire) a bike right on site. I forgot the price, but when you're thousands of miles from Texas, any ride will do.
Swinley has awesome trails. As you ride through you'll think to yourself that there is nothing technical to ride, but take a look around and you'll see some awesome jumps and other diversions. In addition there is a BMX-style track if you want to get some air.
Swinley Forest was the runner-up for the location of the 2012 Olympics mountain bike course.
Location:Bracknell, which is about 20 miles west of London. Swinley forest is located by the Look Out Discovery Centre which is on the south side of town. You can grab a cab there or walk the ~2-3 miles from the Martin's Heron train station.
Directions:Follow the signs to the Look Out Discovery Center, it's on B3430 south of Bracknell and west of A322.