I've been to USA Star Nails at Clapham South a few times and I'm never disappointed. Friendly staff, huge selection of colours to choose from, and overall positive experience.
I went in with nails that hadn't had any grooming for months, and came away with a pristine two week manicure that so far hasn't budged. The service was fast and friendly, and I would definitely go back again!
Threading was good for what I paid for. Only issue is that the skin where the waxing too place is now quite tight and itchy and I don't normally react to wax.