that's interesting...I never knew they had seperate licenses for automatic/manual. In the states you take the test in whatever car you please and get a standard license that's good for either. you don't need a test in manual in order to drive one.
Well, if you take the UK test in a manual car, your licence will cover you for both automatic and manual cars... however, if you take the test in an automatic car, you will only be licenced to drive automatic cars, and you will not be legally allowed to drive a manual without re-taking the test in a manual car.
If you can, I would see if you can take the test in a manual, because then you will be covered to drive both types of transmission (which may come in handy in the future as there are so many more manual cars in the UK than automatic... and automatics do tend to be more expensive).