Thanks all!, If I need a new SIM to be first activated in UK then sent to me in Australia, - and if this is not illegal, then it should be possible for someone at Virgin Mobile to do me that favour and to send a pre-activated SIM to me.
It appears simple for the "tech-savvy" to activate a SIM, - and I would ask one of my children, or grand kids to help, but they live in Australia (that's why we are here to visit). My friends and relatives in the UK are all elderly and probably use phones locked to a service provider. Though it sounds easy, anyone I ask may be confused by the SIM swapping process.
I appreciate that the need to activate a SIM in the UK was probably introduced for security reasons but since it is apparently easy to circumvent that purpose it would now be helpful to offer a pre-activated SIM to people in my circumstances. I would be happy for any costs to be put on the monthly bill.