Global-3 (G3) SIM card from Hologram (this is the latest, cheapest to use and most powerful SIM they offer). Please see hologram.io for their pricing and setting up the SIM. It is very easy to setup and use with little configuration. For trackers, smart metres, remote cameras, sensors, etc - works with any IoT device that needs a SIM card. It is triple cut SIM so can be used as either a standard, mini or nano SIM. It can switch networks (eUICC), is global and in the UK uses all four networks (O2, Vodafone, EE & Three). Please see their website to confirm this.
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