Hi Reddeagle,
I think the first question re location of company should go to the uk agent(s).
What I hear back from a friend in the uk contracting is he had to set up
a UK co, reason was the end customer/agent insisted on it (they didn't want to be billed from an irish ltd co). So this may dictate.
On the IR35 side, he seems to be ok through a combination of regularly changing contracts and billing to/working for more than one customer. I guess this makes sense; if he's only there 3-6 months and has other customers, he's not seen as an employee. The expenses rules do seem to be more strict too, from what I gather, more along the lines of fully receipted, and with fairly miserly mileage allowances.
hope this helps somewhat.