In the Covent Graden area alone there are three or four shops within walking distance of each other.
I've just bought a new bag and I found the shops in London very helpful indeed - my advice is to go at a quiet time, during the week when you are more likely to get an assistant with experience. Take your time and don't be pressured into buying and of course try it on and walk about with it.
It's difficult to get both good advice and variety of product ranges in the same shop in central London - itchy feet certainly seems worth a look but I'd also visit Cotswold Outdoor on Piccadilly (near P Circus tube and opposite Waterstones)
As John said there are three or four "outdoors" shops close together in Covent Garden which is good when choosing as they all have slightly different stock. Three on the South and a very big Snow and Rock just to the north.
We tried them all out when my wife was looking for a backpack and found that the one that suited he best was from the North Face shop, about 30 litre. It did not have the greatest stock just the one that suited her.
Peaceable Projects Inc. is a U.S.-based non-profit group that brings the vast resources of the wide world together with the ongoing needs of the people who live, work, and travel on the Camino de Santiago pilgrim trail network in Spain.
At the risk of sounding like a demon 'less is more' type - I really would not have enjoyed my introduction to the Camino in May with anything more than a small and light pack. It took seemingly endless packing and re-packing, weighing and disgarding unnecessary stuff - but I really didn't find there was anything I missed - and we encountered cold nights, hot days and even torrential rain! My pack, a waterproof liner for it, and walking poles were the biggest expenditure - everything else was from charity shops or homemade.
I took a Berghaus Freeflow III 20 litre pack (£32 on the internet) and the final pack weighed just under 6.5Kg without water. It allows airflow between you and the pack (heaven when you can catch a breeze to cool you down), and although I have a small frame, it fitted perfectly. But even this amount of weight really makes its presence known on a tough day - and I would wince at the grim-faced folk heaving larger packs on after a break.
But as everyone has said - shop at quiet times if possible (I tried mine out in Millets, but bought online because I was picky about the colour - sorry Millets!), and if possible take a heavy bag of shopping with you to check what it is like with weight in it.