In the UK, one can usually drive with their comprehensive cover any other vehicle with 3rd party as long as that vehicle is already insured through its owner and you of course have permission to drive it. If an expat with Vollkasko cover pops back to the UK and a friend offers to lend him/her his car, would the Swiss motor insurance cover at least the minimum legal liability? Seems a shame that I can't take up an offer of a free car over Christmas when I'm paying through the nose for my comprehensive Swiss insurance....

This is a question you should ask your insurers about. Without knowing the terms of your policy I don't see how anyone can give you the answer.
On a UK policy the extension which provides third party cover for a vehicle not owned by you normally applies only within the UK and not abroad.
Why not just ask your friend to get you included on his insurance for the period of your visit ?