I wanted to upgrade my iphone SE to iphone 8 and I am with O2. I couldn't find a sensible deal O2 seemed to want to put me on a 36 month contract so I opted for a sim only deal to hold me until I made a decision.
I received a mail from Mobiles.co.uk which had an offer for iphone 8 with 10gb data unlimited minutes and texts for 24 months at around £6 a month less than O2 (12 months less contract) all still on the O2 network.
Simplicity itself to switch on the website and within 30 minutes O2 had confirmed all the changes new phone arrived next day.
Great service and simple to use the website.
Still with O2 and very happy with the deal achieved.