Red cuádruple del Reino Unido, 25 GB, 365 días

Great sim with data only for travelling around Europe and UK, with a 6GB data plan allowance, it can connect to all the networks, allowing you to receive a good signal wherever you are.

These work without registration (though you can register it to check remaining data or auto-renew, which I didn't personally need). Just put the sim in your unlocked device, wait a minute or so, and you're good to go on whichever network offers the best signal for your area. It works great as a wifi hotspot in my home, for backup when the main fibre optic internet goes down. If you have a modern/posh router, you can use this alongside your main hardline internet, and it'll auto switch to the 4G/5G sim when it goes down. Very useful to keep permanently connected, though obviously you'll need to watch what you download when this occurs, so you don't go over your data limit.

Other use cases would be for foreign visitors to the UK, who just need a data sim to keep connected on UK-based networks while on holiday/working. It also works in the EU I believe, so UK peeps could buy one of these while in Europe travelling, avoiding any nasty roaming fees on their domestic plans. Let's be honest, most of us use internet-based chat/calling rather than SMS/calls now, so the lack of call/message support is not really an issue any more. Data is king, and this sim provides.

These are also much cheaper than other data-only sims I've used, and more versatile due to the multi-network function. Good stuff.