The IR is all about procedural instrument flying and it's considerably cheaper to do it in a single. That's why Bristow use a single in their instrument school in Norwich. After that, you have your IR and can just do an upgrade flight in a twin later. Incidentally, the Bristow instrument school at Norwich will become part of the Bristow Academy (formerly HAI) and should also have twin AS355s available for twin engine instrument flying later this year.