Yes, I've had a Mini too. Parts didn't get quite the same quality control back then as they do now. Of course, I'm sure if you're building from the ground up and have a box full of parts to choose from, then you can lose a few kg, but there are not enough Roadsters about to have a stock of parts to compare. I'd say that's pretty low down the list of things to do.
There's weight to be lost in the Roadster, just not as much as you'd expect. Some components are just plain heavy, like the roof bars. When I looked at what I could reduce weight on, there were no big gains to be had other than the seats as everything was already remarkably light. Of course, if you're building a track car, you can pull out lots of things you don't need, like aircon and if you're willing to machine new aluminium uprights, you could save a few kilos there too. Small gains though.