There's a problem with the likes of
Evans and
Halfords - neither will swap stems/saddles/tyres if you either want or more importantly
need the former two items to improve fit/comfort. Poor service really.
I'm willing to bet they'll sell you an upgrade if you want or need one though. Or even better, let you take the bike you bought, then over time work out which saddle is better for your needs. Then you can explore a whole new world of parts outside of their limited range. It isn't poor service, it's a business model that sells an enormous number of reasonably priced, well equipped bikes. The shops that swap the saddle or stem etc won't be doing it as a public service, they'll factor in the cost to the overall price.
What we tend to forget is that the average cyclist or wannabe will buy a bike and adapt to it. Not everybody wants a micro-measured bespoke fitting from the off. That service is available to the enthusiastic amateur who is at a stage beyond the first time Halfords or Evans buyer. So, if you want to pay LBS style premium for that experience, then go ahead. I bought a stock roadbike at basic price, expecting to learn over time what I needed to fix to make it right for me. The saddle was the first thing I changed, but again, I knew what would suit (the old Spoon off my last roadbike) Had I asked the LBS to fit a spoon I would have expected to pay RRP for the upgrade.
As an enthusiastic lover of building my own custom MTBs, the last time I bought a bike in an LBS (other than a C2W roadbike) was 2008. My Boardman Hybrid C2W bike in 2009 was of course Halfords, but I enjoyed the incredible value for money, and happily fettled it to suit me over the first few weeks of owning it.