SuzanneC1 - The One Plan's main advantage over other Three plans is that it includes the ability to Tether (ie use your mobile as a hotspot and let another device or devices such as laptop, tablet, or a mobile with limited data allowance, use the Three mobile instead of a wireless router at home/ office).
I know various Apple and Android smartphones are available on the One Plan but some can be used on other SIM only contracts (unless you are always wanting to switch to a new phone each 18/ 24 months).
You can get unlimited internet on several of their other deals, either with a phone (eg Samsung Galaxy Ace, at 13/month) or from a SIM only deal.
They have priced the Ultimate Internet 200 (200 minutes) at 12.90 and the 12 month Essential Internet 600 (600 minutes) at the same price. However, if someone only needed 200 minutes, they could get the Essential Internet 200 (12 month) SIM at 6.90 and for 3 quid add all you can eat internet.
I know these don't compare with the One Plan at 2000 minutes, and 5000 minutes to other Three customers, but I suspect they put massive figures on to encourage people to spend 25 quid a month, when they perhaps use a fraction of those minutes (I know I have plenty left from 600 most months, and I speak to 3 sisters and various friends most weeks!)
Only pointing it out so when your One Plan contract reaches 24 (?) months, you can consider whether to renew or go to a cashback site and get money back while starting a brand new Three contract. (NB you'd need to ask for the PAC and switch for a few days to another network - eg get a 5 quid ASDA SIM and move your number to them, then start a fresh contract with Three via a cashback website, and move your current number back to Three.)
Just checked and Quidco are offering 41.67 cashback to someone signing up for the "Essential Internet SIM only 200" 12 Month (that's the one at 6.90 a month, so it is like 4 months free based on spending 9.90 a month, if one added the 3 quid 'all you can eat' internet option (need to do that after SIM is active by talking to Three).