I've been to both first encounters and window to the womb for reassurance scans. Can't fault either establishment, however first encounters do not do internal scans only abdominal so depending on how early on you are you might be better off with window to the womb instead. Personally would reccommend and early scan especially if you are anxious, unfortunately my first reassurance scan did not have a happy ending but I'd rather have known that at 8 weeks than waiting until 12 to find out the same horrible news. Second time round the reassurance scan really helped me feel a little better but after our history the effect didn't last that long!
I didn't get a reassurance scan as such but I paid for an early gender scan with First Encounters. They were brilliant, they took their time with the scan, I came away with loads of pictures, a couple of videos (some packages they do include them), they got the gender correct at 16 weeks, and honestly they provided loads of reassurance that everything was good even though that isn't what it was for. If you're feeling anxious about it, I couldn't recommend an extra scan more. It really did settle my mind between the nhs ones