Loose_Cylic,
Although you might have the right 'ticket', you might find that you struggle with some of the other requirements. I'm thinking of things such as a lack of low level experience (especially for Police Ops) or the lack of experience of JAA regs and requirements.
You might also find that UK airspace is far more congested than that which you are used to (there are an awful lot of people on this small island) and you might even struggle if you haven't been already been exposed to the UK emergency services culture/environment.
And, finally, to answer your NS question: yes, you would need ATPL theory. IIRC, the NS operators aren't really interested in 'just' a CPL/IR because there is no scope to promote you to Captain in the future. As already discussed though, a CPL will suffice for most Police work.
But the best of luck in your hunt,
B73