I would not place much reliance on the staff of Central London outdoors supplies stores. Because it's darn nigh impossible for anyone to live in or around London on a shop assistant's wages, the stores are desperate to take anyone on, consequently the posts are largely filled by people from all over the world, particularly Eastern Europe. Sometimes they're keen, more often they're not, but what is particularly lacking, apart from a complete command of our language, is any real knowledge about the goods they're ringing up on the till.
There's a large store next to a big supermarket in a Buckinghamshire town, where I looked when I wanted some new boots. I spent 15 minutes looking at every boot on display, while the two middle aged female assistants leaned against the counter nattering, as if I was invisible. I then felt that even if they did approach me, or if I approached them, I wouldn't get the advice I needed, so I then drove around the M25 to Cotswold in St Albans, where the guy was excellent. He asked what sort of walking I did and where, he measured my feet, and sold me a pair of boots that I have been really happy with.
As for Millets, I wouldn't shop in there again. I couldn't see what I wanted, a kid asked if he could help, so I asked, "Where's your NikWax please?" He looked blank, another person asked him what I wanted and the kid just shrugged his shoulders. He may have been new, he may not have been familiar with the products, but I would have expected him to be capable of repeating the simple two-syllable word I had asked for.
So, I don't know which side of London you're on, but my advice is get your Yellow Pages out and head out of town where I think you'll get proper advice. If you're on my side, the north (I don't know about south of the Thames, I don't cross the river!) there's a few scattered around Hertfordshire, particularly St Albans for example. Good luck!