I'd highly recommend The Fetal Medicine Centre on Harley Street. Have had scans there for all three of my pregnancies. They take blood and perform a very detailed nuchal scan with immediate results. The professor there Professor Kypros Nicolaides was simply amazing when I had possible complications with my first pregnancy. He only sees the 'problematic' cases but all the other doctors, in my opinion, have been top notch. A nuchal scan costs £180 and all profits are put back into the foundation for research purposes.
http://www.fetalmedicine.com/fmc/