Alright, went and saw two tyre/wheel stores today.
First, I went to JAX Tyres in Liverpool to ask about the hub-centric rings. He said that hub-centric rings aren't made for steel wheels and wouldn't be necessary even with the ~5mm difference. It would be a different story if they were an alloy. That was the end of that.
A few stores down is a Bob Jane T-Marts so I thought I'd pop down there for a second opinion. This guy was much more cluey about off-road stuff but he echoed the first guy - hub-centric rings aren't made for steel wheels, and he said it'd be useless and that installing them would adversely affect the car. He asked if I was experiencing any vibrations (which I'm not) but said not to worry about it and that there shouldn't be any sheering of the lug nut. He also said that they're only to centralise the tyre, and not to prevent shearing. Spent a fair bit of time talking off-road stuff and he showed me his Prado, too
![Laughing :lol:](./images/smilies/icon_lol.gif)
He did say that during the week he works for Speedy Wheels, and that their steel wheels are 106mm.