Your GP will give you a referral. The 12 week scan is to measure the nuchal fold. You also have a blood test a few days before. You don't have to have the test done but it's recommended for women over 35 who have a higher risk of DS or other trisomies. You get given a risk factor say 1/10,000 or 1/200 etc and will be told if it is recommended that you have further invasive testing.

If you don't want to have this test then you wait for the scan at around 20 weeks. This is the one where you get all the measurements done.

I can't help with where to go as I'm on the other side of town. Some places will do 3D/4D, some don't. Your doctor will probably give you a referral for a certain place anyway.