Hi Michael..........you are correct.
Another stipulation on driving other vehicles on a third party basis is you usually have to be over 25 years old. I'm insured TPFT on my Audi and I can drive other vehicles on a third party basis i.e Hazel's Audi.
As you say, I can't drive the Golf on that basis as someone needs to insure it, hence I had to take out a seperate policy.
Hope that makes sense???