Just saved almost £125 over 24 months by adding my son to my O2 Family plan as I received a 20% discount on the 5GB airtime plan on a Samsung S8 Like New.
All you have to do is text LOYALTY to 21500 and within minutes you'll save.
The offer is available on all non promotional offers and I didn't realise until I checked, but you can get this deal on their refurbished Like New phones too. I think the refurbished phones are only available online though, if you're interested here's the list of refurbs:
Hope this helps, happy saving