Most Halfords stores don’t keep anything in stock above £1000 so that model sells less often as it relies on the customer doing some research online and / or the store staff being familiar enough with the range to offer it as an upgrade on the standard carbon model (currently £800 btw). You can check availability via Halfords website and if the bike is in stock centrally get it delivered to your local store.
Size availability (lack of) might be why people end up with the ‘wrong’ size rather than what a store has in stock as most stores (as described above) don’t stock this model anyway. Worth noting that Boardmans are typically a bit short in the top tube though.
Main issues with the carbon frame is poor fitting of the PF30 bottom bracket. Ask for it to be taken out and re assembled with carbon paste as that should deal with potential problems. Encountered the same issue with Felt frames so it’s not exclusively a Halfords issue.
HTH