so what happens if you are lent a car for a few weeks to use?

you simply don't tell the insurance company it's someone elses car, just say that you wish to insure "x" and where it is registered and if it's going to be kept at a different address

at least this used to be the case, if things have been tightened up in the past 10 years them i stand corrected, otherwise how else would you insure a car you've just bought that's not registered to you?