I was looking at signing up for the 9gb data for £8 contract that @crazy_b
posted last month, and noticed that you can actually get 10gb for the same price now! Thought it might be useful to anyone else that would prefer using O2 over other providers - they're the only ones that supposedly give me good indoor coverage at my house, hence why I am switching from Three even though I am getting slightly less data now. But with the new lockdown, I doubt I'll be off the wifi for a while anyway! Hope this helps someone else too.
Edit: this now includes 6 months of Disney+ for free!
Edit 2: Thanks to @Marcus_72
for adding that you can get £20 off via TCB or £22.50 via Quidco, which means total cost could be as little as £73.50 for 12 months (or £6.13 a month!).