With regard to a foreign aircraft operating in the UK for money, ANO article 138 is the bit I think is relevant. Which amounts to get permission from the secretary of state (I think for transport) and off you go.
I don't know if thats what went on here, but it is going on in the UK now, with some European AOC holders.
Pracitcally I believe the permission comes from the DofT and the CAA don't have a great deal to do with it, but for being notified and having the right to check up on it (don't quote me on the last bit).