I for one wouldn't let
Halfords anywhere near one of my bikes. The odd time I go in there for car-related stuff all I see in the bike section is a bunch of chimps hanging around with their fingers up their arses talking to their mates and doing little else.
Bike Hut at St Pauls are an exception but they seem to operate like an LBS.
On a side note I'd be fairly sure that bike manufacturers have a strict retail pricing policy so how much can web retailers vary over an LBS? Not by much I imagine. Any sale price is usually funded by the manufacturer to clear stock for next season's. The biggest difference I see is that online/mail-order tend to be the guys that offer the 0% finance deals.