Honest Mobile is known as one of the most ethical and environmentally sustainable mobile providers.
It's a carbon negative company that offsets double the carbon footprint cause by your smartphone, and donates 1% of bill revenue to reforestation schemes.
Honest Mobile uses Three's mobile network, which reportedly covers 99.8% of UK homes. If you live in a fairly urban area you will likely be able to access Three's 5G network while out and about (and potentially at home) too.