Change region on your account, buy the game on the eShop. Change back if you want to have higher prices again.
Use this to find the best region for the game you're interested in:
https://eshop-prices.com/prices?currency=GBP
The price you pay depends on your card's exchange rate so use a Halifax One or Revolut card to make the purchase. Canada, USA and Mexico are a bit of a pain to buy from but I've had no issues on all other stores.
No, you have gold points on both the UK store and the SA store. You should always be using all of your gold points for every purchase anyway so it you should only ever have a few quid's worth on any of the stores.