I think Harmony might be worth it if you are high risk for Downs. As it's less risk to the foetus than an amniocentesis.
Otherwise I wouldn't bother myself, but I know some people like the early reassurance.
Bair you can get a private scan prior to your 12w dating one, you know? I think early 'reassurance' scans are about £50 where we are, through BabyBond. I didn't have one myself as heavy bleeding in week 6 meant I got an NHS EPU scan - don't recommend that as a scan-obtaining method though 
Thanks for kinds words on BP, everyone. I was in bits a bit on Sat night as I have a real, real fear of gynae procedures and was feeling so much more relaxed about the birth at home. Plus our hospital locally is not the best (sky high intervention rates, not enough staff etc). But what will be, will be, and as long as the baby doesn't get injured during the birth, that's fine.
I am mostly worried about the baby getting hurt (this happened to someone I know due to ill-advised forceps delivery) and also getting PND if I react badly a highly medicalised birth (as I tend to get weird anxious thoughts and dreams and upsets over gynae stuff so kind of worried it will be even worse with something as dramatic as birth). I would just so hate for my baby's first weeks of life to be this mental black hole for me because of some psychological reaction - this will be my only baby, and it's taken so long to get here, and I really just want to enjoy and feel good during her first weeks. It's time I'll never get back :(
But then, hey, who knows. BP was 140/98 on Sat, which is mild hypertension, and then 119/83 today (83 not great but within normal range) so we will see tomorrow. Trying to stay caaaaalmmmmmm! 