Just looked at the price on one of the websites that I buy equipment from in Thailand - other places may be cheaper. The price is 16,800 Thai Baht - at current exchange rates 382 UKP, and the exchange rate is very low at the moment - has been ever since the Brexit referendum. Even so, this is slightly better than 689 UKP. If the exchange rate can get back up to 50 again, it would only be 336 UKP - less than half the UK price.
https://www.digital2home.com/shop/canon-flash-speedlite-600ex-ii-rt/
Just one of the reasons why I turned my back on rip-off Britain (there were many).
Dental work and medical procedures are very professional in Thailand, and a lot cheaper than the UK. I reckon that if someone in the UK needs some dental work done, or a scheduled medical procedure, and also wants to buy some camera gear, the money you will save in Thailand will probably pay for a free vacation. I also pay significantly less for petrol - about 61 pence per litre. I don't know why Brits put up with it.