Others have commented on the SIM only thing and "new customer".

Few things:

1 - O2 is garbage, just incredible garbage, in terms of network coverage and data speeds in the non-metropolitan UK. Vodafone similarly is like going back in time 10 years in coverage compared to EE. Three is a joke. If you never leave London then O2 is perhaps acceptable, but if you move around then you should be on EE. I understand that in the past the O2 roaming proposition was compelling but it's not obvious to me that is the case now.

2 - Always compare estore avios, and its very unreliable tracking, with topcashback/quidco.

3 - Moneysavingexpert will have information on mobile contract prices that is decent although not necessarily the best achievable. Hotukdeals usually has good threads on SIM-only deals and the comments will show where the best value is. Sometimes the best value is retentions deals - I'd pay a *little* more pcm to be in SIM-only contract for 12 months vs 24 given mid-contract price rise and waiting a year longer to be able to deal with retentions.

4 - If unsure of your ground, pay a *little* more to deal with the network operator direct rather than an MVNO (so O2 not giffgaff, Vodafone not talktalk - but see point 1 above) as the MVNOs sometimes have unpublished or hard-to-find limitations (fewer bands available, no 5G, speed limits, deprioritised traffic, or notoriously in the case of giffgaff the entire IT operation being lashed together out of car-boot-sale castoffs).