Not something you'd expect to say, but it looks like for both monthly cost and call charges Andrews & Arnold are cheaper
https://www.aa.net.uk/voice-and-mobile/voip-informat...
I disagree, Voipfone is a better choice and likely to be cheaper. You don't need a call package as call usage is unlikely to exceed the monthly fee. Calls between Voipfone subscribers are free as is a call to Voipfone's help desk. Select a phone number from what is available from Voipfone and that is free as well. Retention of the number after this first month costs £3.60 pm. From scratch, you can be up and running in under an hour with £2 call credit thrown in. It's a PAYG service, only requiring a call deposit to be made for outbound chargeable calls onto the public network. There is no time limit expiry on any deposit made but strictly no refunds if you discontinue with the service..
There are porting fees payable for a number, inbound and outbound
Registration is tedious though, but once you have jumped through all the hoops, the service is easy to setup with any SIP hardware you wish to use or via their softphone app. Voipfone will hold your hand with the setup if needed.