I also recommend the FMC. Pretty much one of the best, if not the best place in the world to go to get it done as they are pioneers in fetal medicine research, and probably the cheapest too even without the special deals they often have. And they are lovely 
If you know you are going to get Harmony done (i.e. you need to know for sure at 12 weeks) get it done before your NT screening. Reason I say this is if you get a worrying result at your NT screening, you are left with the unenviable choice of a) waiting two weeks worrying yourself stupid or b) invasive testing and the risks that carries in order to find out within a few days.
Whereas if you already have the Harmony result, neither of those options will happen.
I went at 10 weeks for my Harmony blood test (had viability scan first) and back to the FMC for the nuchal ultrasound at 12 weeks. I am so so so so glad I had the Harmony beforehand as my NT screening came back at 1 in 5. I was reassured before the NT screening began that Harmony had come back low risk and any result I had from the FMC NT screening was to be ignored in favour of the Harmony. I think I would have passed out from shock had I not already had the good result.
The research doctor was very reassuring about how Harmony could be trusted over the scary NT result - she was very very certain my baby does not have Down Syndrome. Yes, Harmony is a screening test, not a diagnostic, but being an obsessive and having looked at the research statistics myself in detail I can understand their total confidence in the results. I'm 28 weeks now and have never had anything slightly worrying on ultrasound (anatomy and cardiac scans done at FMC too) to suggest there is a problem - at least half, if not more of babies with Down Syndrome show something on ultrasound.
I think FMC like to do the NT screening anyway for their own research purposes to compare against the Harmony result, but it's not compulsory. There are still benefits to NT screening that Harmony does not give so it is still worth doing one wherever you fancy doing it. An increased nuchal fold can be an indicator of other non-chromosomal issues which can be checked up on, and a low PAPP-A or a high bHCG from the blood screening can be indicative of future placenta issues and would warrant a third trimester growth scan.
Good luck!