"""The pick of the bunch for me is Three's One Plan"""
However, you don't point out that Three do not offer visual voicemail - where you can see a list of the voicemails you have received, who called you, when and how long the message is.
Instead, you have 90s style voicemail - "Press 3 to delete this message, press 7 for the next message, press star to accidentally override your greeting with 'erm, what did I just press?'".
Well, it pissed me off after I switched from O2 to Three. AFAICT, only O2 and Vodafone have visual voicemail.