Restore on Camden/ Wexford St is really good (at least I am nearly sure it's called Restore...it's just up from the Palace nightclub on the same side of the road).
Holland & Barrett can be great value, but offer zero personal service or product knowledge.
Restore is quite small, but they have excellent knowledge of their products, and are great at sourcing stuff in for you if you need it. They are no dearer than H&B either, who up the prices of their single packs to make their multi-pack deals look like better value!!