Our recently inherited roadster doesn't seem to have a jack in the boot. What does everyone use (to carry in the boot)?

I see the original metallicraft type is a scissor jack with a pad on it (there is one on ebay at the moment). Can I use any jack so long as it fits under the car? I'm just talking about changing a wheel by the side of the road, not for working on the car. I realise also that the original jacking points aren't brilliant apparently and so any recommendations for a roadside set up and where to jack up for wheel changes would be much appreciated.

Thanks in advance for your help.