Ps4 – If I purchase PS Plus, do I get access to old PS Plus free downloads on another account

Unfortunately, you can't do that directly off the PlayStation Network Store, since the buy option won't be available for games you own through PS+, only after your subscription has expired will that option be available again.

However, you may be able to purchase the full games through third party sources. For example, GameStop offers download codes for PSN games that can be redeemed on the PSN Store. I assume that if you redeem a code for a game you only "own" through PS+, you'll own it permanently (you can check the game's information in the XMB and see if it has an expiration date).

I cannot confirm this method 100% as I haven't tested it myself, so try at your own risk.

No. Content licenses are bound to the account that activated them. Once the PSN+ account laps, it defaults to a standard PSN account, the free games that were licensed would then be invalid on that system for all accounts. But, any games you had actually purchased, even at a discount price would still be available for all accounts (unless you also removed the account from the system).

The only way to be able to play those games again would be to either renew your subscription to the account that originally downloaded them, or just buy them under the alternate account.

Specifically in regards to your question though. Any "free" games that you downloaded on your UK profile would have to be re-purchased by the US profile to re-aquire them. The only way you would be able to have them free for the US profile is if you had also had a paid US account concurrent to your UK account and had activated the game when (or if) it had been available for that account.

I know you may think this is stupid but remember, the free games are a perk for paid subscribers for that region. If you no longer subscribe to that region then you are no longer eligible for the perks.