I'm teaching myself some small electronics projects and looking for a network in the UK that will allow me to top up by e.g. £5 and leave it for as long as possible - likely 6 months, before having to top it up again to keep it active.

It will be monitoring and sending data back in tiny packets every month a few things.

I can remember Vodafone used to do it, but not sure if they do anymore.

Also, I might need a couple of them in different locations, so if any networks provide a SIM for free, or as close to (e.g. £1) that will stay active.

Are there any? EE don't. I don't think. They deactivated my SIM before I even got to top it up.. 😁