Is your house unusual for the UK in having air-con? Or do you use a dehumidifier set on a very low threshold? This certainly shouldn't happen for any normal humidity level, but if your house has a particularly low humidity level then you might have problems. I know folks in the US (who almost universally have air-con in their homes) often have to use humidifiers in their guitar cases to prevent problems, but air-con in the UK is unusual (and mostly not required; come to that it's not required in much of the US either, if they'd just open their windows and get some airflow, but never mind).
But if we can rule out your home having created the problem, then this is a pre-existing cockup by the instrument makers, so take it back to the shop. If there's any binding on the edges of the fretboard, you may end up splitting it when you hammer the frets down. And once you start hammering on the frets, you can forget about the shop taking it back if you do find you can't fix it. The shop will likely have a returns policy with the manufacturer/shipper anyway, so they shouldn't lose out.