From my recent experiences based on testing out a few different mobile SIMS.
If you are a BT or Plusnet customer, there are some good SIM only deals, and it's EE network.
However you don't get all the features of the EE network, BT does give you Wi-Fi calling, last time I checked Plusnet didn't.
I tried Voxi which is owned by Vodafone, fantastic value for money £10 for 15Gb, best PAYG deal in my opinion. I found Vodafone 4G speeds pretty good.
Recently moved to EE on a 12-month deal, £10 for 10Gb, good deal if you want EE network and all the features. I think the deal is hidden on the EE site, this is the site I used.
EE 10GB For £10 (12 Month SIM Only): With 5G & 6 Months Apple Music