In my own experience all the Hermes stores in London have differnig strengths and weaknesses;
Harrods - I would avoid like the plague no matter what, it is always crowded and for the items they routinely stock just not worth it. They occasionally have rarer items in teh back but it takes them ages to get and they do everyone is trying to buy it once its brought out front. If you can track down an item though they will sell it and not often hold it back. Just don't go in with high hopes, they sell out of the good stuff!! within minutes of opening although tehy can get deliveries at any time.
Selfridges - The new concession is perhaps 2.5 times the size of the Harrods midget store and stocks a wider variety, however, bar silks and costume jewellery, most leather goods are usually reserved for their regular clientele as they seem to get the least deliveries in London. However, there are a few very nice SA's there who can sometimes be obliged to offer their help! If you can get theri attention that is
NBS - Is far more of a walk-in store and the clientele base has a reasonable percentage of tourists, if you want a b/k/c this is your best chance, even if in general I would argue getting a bag in London is possibly one of the hardest cities in the world to score in, particularly at the moment. They have massive stock and if it is not on show just ask as there is usually something in the back. They may not offer it though

Sloane Street/ CP - This is my personal store of choice and I have built a relationship with at least 3 of the Staff there. This is in general the store for Londoners and it has a strong clientele list. The new staff are so much better than even a couple of years ago and more approachable. However, I would argue this is the hardest place to buy a b/k/c as they are all reserved for clients and even getting onto their wishlist is a miracle. It has great choice and they are very open about ordering things in, which is unusual.
These are just my opinions and I am a regular client at SS for at least 3 years, so this maybe biased in my preference. I hope this helps for those who are looking at the best places to buy in London - it is indeed hard without a relationship.