Congratulations! As a previous poster said you can walk in for free to the St George's EPU (I think Thomas's also has one if that is easier for you) and they will scan you. The clinic runs every weekday morning. There can be quite a long wait and they will scan people with pain and bleeding first (understandably) however I believe they usually get to everyone in the end.
If you are intending to have the Harmony test then the Fetal Medicine Centre on Harley St does a package which involves a scan at 10 weeks, bloods taken for the Harmony, then a further very detailed scan at 12 weeks when you also get the Harmony results. This package costs £400 which is less than many other places charge and the FMC are absolutely the scanning experts. So if you can bear to wait till 10 weeks that would be a good option. Not local though obviously