I use this:
Three Unlimited 5G Data SIM - Preloaded Every Month from now until 8th December 2025 - Perfect For Wifi Routers, Tablets & Phones -
https://amzn.eu/d/elXSlIb
Works out a £7.40 a month.
Or this:
Three Unlimited 5G Data SIM - Preloaded Every Month from now until 8th August 2026 -
https://amzn.eu/d/iR6b8BO
£7.30 a month.
If you want to stick with O2 then:
O2 Unlimited 5G Data SIM. Perfect for unlocked Phones, Routers, Tablets and Wifi Dongles - Activate anytime when you receive the SIM - Choose your Data Duration (12 Months) https://amzn.eu/d/aK0loL6
Is a bit more expensive at £19 a month.
All are payable in advance but that's one of the reasons you get decent savings.