Decathlon ebikes are manufactured in, and imported to the UK directly from, China so Brexit is not a factor (their non ebiles are assembled in Lille, France).
Demand is at a record high for recent decades, and manufacturing is at a record low for recent decades. It's not difficult to make the connection.
Not only that, but production of everything from the raw materials right through to the finished components required to build the bikes is also down, which pushes up parts prices before anyone even thinks about bolting them together to make a bike.
International freight prices are also on the up for various reasons, which won't have helped matters.
And while I dont think they're profiteering, they are in business to make money and doubtless the British division, which operates independently, will set their prices accordingly. In addition, bsuiness rents, rates and taxes makes the UK one of the most expensive countries in the World to be a retailer. As such you cant really compare to how much the same model costs in Timbuktu and come to any valid conclusion.