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So I can be in my UK account and play from the US account? I don't want to be in separate accounts in order to play games
Yep, that was always my biggest worry. You need to activate the code on the US account and install it first there. Then log off, come back on as UK account and the game will be playable. You need to make the PS4 your main console on the US account through the website, but having the same console as main on both UK and US accounts does not seem to cause a problem. There's plenty of guides on Youtube and it does work. Once setup, it's also nothing like the amount of hassle it sounds!
I gather there may be some issue regarding DLC in that respect, but I've never bought any so I can't comment. If you want to buy from the PSN store, buy a gift card first using the overseas Amazon account, add it to your US PS account and buy what you like. I bought BF4 and Metal Gear that way a couple of months back.