@Tez_20 In the UK we have to be careful with anything we add to the tank, from dechlorinator to medication. Most of them give the dose rate in litres and gallons, and it's usually US gallons. But the odd make (King British springs to mind) uses Imperial gallons. It is much safer to measure the dose in litres. I discovered this long ago when I realised that King Birtish dose rates didn't seem right for the gallons rate.
I tend to look at bettas in just one shop as they usually have a good choice, and I let the betta choose me rather than the other way round. I don't go shopping with a type or colour in mind - I look at what's in stock on that day, look at which are perky fish with no obvious health issues, narrow it down to 2 or 3 then decide. I don't mind what they look like as long as they look fit and healthy