The Fitbit Charge 2 is available at a good price from Dixons Travel.
There is a double the difference money-back guarantee if they are beaten on price by Amazon, John Lewis, etc. Returns can be done through the high street.
I got one for my son at Luton Airport, so I think the deal is available at all Dixons Travel shops.
The only downside is that you need to be flying to get one!