I think the main reason (like previously mentioned) is the fact that the UK CAA don't want people to be careless about flying into IMC conditions in a VFR single engined machine just because they have an IR and feel confident.
A load of bull if you ask me. I have a mother, I don't need a second one. The CAA are just being cowards in the face of (in their view) potential litigation and I presume feel that as newly minted IR rated pilots we will be overcome with childish excitement and take our R22s straight into the nearest cloud! Admittedly there might be a few, but legislating for the minority is paranoid at best.
Just to add insult to injury, even though we are in 'JAR land' and we 'supposedly' have the freedom to get our training and ratings in any country that is JAR compliant, if you get your SEIR(H) in a country that allows it (like Sweden), the UK CAA will still refuse to put it on your licence. So much for Mutual Recognition!
There's a word for people like that, it starts with 'F' and ends with 'uckers'..