As far as I know it was not legally available in the US.
That isn't necessarily a bar to a US court taking the case - the barrister example of a snowball thrown near a border may arise.
However he is time barred in the UK, unless he can show that he could not have known about (whatever he thinks he can sue for) until over a year later. Considering it was a national TV broadcast a claim like that would not impress anyone who was formerly in awe of US intelligence.
The key thing he'd have to prove is that the program - and the program alone - either caused him a financial loss, or a reputational loss. Since he won the 2024 election it would be hard to show a material loss. And plenty of people called him an insurrectionist cunt before the program was aired. The BBC are welcome to use any and all of the posts I would have made pointing this out at the time if they need a hand.