Mobile intermediaries are there to turn network sales commissions into attractive retail packages. How they divvy up "discounts", "free gifts" and handsets is their own business, if they dont do the package you want, go somewhere else.
Your phone contract, yes, is managed by the network
Any warranties for your handset are supported by the manufacturer but some retailers will aid you if required.
Scenario: My bill is wrong - contact your network
Scenario: My phone is buggered - Take the phone, along with your proof of purchase, to any Carphone Warehouse, who'll handle sending if away for warranty repairs at no charge (unless it's a non-warranty failt, of course)
Scenario - My £34573 chequeback deal, where's the cheque? - THEN you need the retailer.
The majority of bad on-line reviews are posted by slack jawed troglodytes who can only barely manage to dress themselves in the morning, let alone operate a mobile phone correctly. Positive reviews are few and far between since the British seem to just expect the moon on a stick, for free, daily. Why go praising people "just for doing their job".