I will be watching this thread with interest - as I'm very much in research / learning mode.
Not long ago, I didn't know what a SIM was (and I'd still rather not know what one is 🙄)
However, I've scoured the comparison sites, taking note of the devil in the details.
I've found Lebara (Vodaphone) was my best choice, when bought through Moneysupermarket.
Of course, your price depends on the amount of data you want.
Even though monthly 'contracts' are available, SIM pricing in general is affected by the annual, inflation + whatever-they-fancy increase trend - conveniently invented by home broadband providers.
I'm currently only on 15GB just to test the signal around the house. (Important as I've just dumped the home broadband provider - with a view to relying solely on tethering).
Consequently, I'm looking forward to reading about the best 50-100GB SIM deals (preferably on one month contract) that anyone knows of.