Eventually, I bought two things...
https://www.ebay.co.uk/itm/254669429644and
https://www.ebay.co.uk/itm/115068034580Have got both in my grubby paws now, so just need to make them work, for which I apparently need summut called a SIM card, whatever one of them is.
After several more seconds of extensive research, it seems that a UK PAYG SIM card is the cheapest option for both the USA and Oz. So I walked into the local 3 shop this morning, and tried to purchase 2x12GB preloaded data SIMs for £15 each. What a faff. Turns out there is some 'summer data deal' on, so the 12GB has been upgraded to 16GB, they're £10 each, and they're on a buy two, get one free deal.
Anyhow, I now have three 16GB PAYG sim cards for £20. Just got to work out how to activate the wretched things now. They only last 30 days apparantly, and have to be activated whilst you're still in the UK.