Mountain Warehouse is certainly an avoid - most of its stuff is fairly overpriced or outright tat (Ok, you can be lucky - my wife bought a light fleece for a fiver which seems to have survived OK). I was chatting some months back with the staff of an about to be closed down Blacks, who were really annoyed that they were closing and reopening some months later as a Milletts, right next to Mountain Warehouse - quite rightly they thought of them as selling not very good stuff, with constant 'sales' to give the impression to the unwary that they were getting some sort of bargain. As someone commented, they look too good to be true! They do much the same as Sports Direct, which is another place which I avoid.
The fact that their staff discount in 50%, when the generous retailer (like Blacks or my employer) gives you 25% says a lot about their wholesale unit price, and hence quality.
Even Blacks would be a better choice, and you can pick up some good deals in Lidl/Aldi or TKMaxx. Personally, Cotswolds is a place which has better stuff, and if you buy carefully, you might not be paying much more money. And Alpkit does far better sleeping bags, etc for little more cash.