Soon they'll be blaming the weather on Brexit.
After Brexit the value of the pound sterling declined against other currencies. This means that all goods coming into the UK will now be more expensive. The price of portable recorders, none of which are made in the UK, will all go up. However, some dealers will still have stock purchased before the currency devaluation. Some dealers will sell them as before, while others might hike the price for extra profit. This is why prices will be different at different dealers. (Besides all the other factors.)
I wouldn't think I'd have to explain such basic matters here. Brexit does indeed negatively effect the UK economy and the price of good. Sorry, but it has nothing to do with the weather.
Sorry but your objections to Brexit don't apply in this case. The Brexit vote was on June 23, 2016. Its effect on the pound had already occurred by the time the Tascam DR-100MKIII was available for sale in the UK in October. UK retailers would have paid for their units in the already devalued currency.
So Brexit is not an explanation for the differences in UK prices of the Tascam DR-100MKIII. And in fact the average UK price is about £349.99. That translates to about $435.00 in US dollars and is very close to the US street price of $399.99. Nothing to complain about.
The jury is still out on Brexit. It's true that a devalued currency can increase the cost of imported goods, but it also helps exporters, making their products more competitive, and with their increased sales, it can actually work to stimulate the UK economy.