If you visit the Statement of Services link from UK Power Networks wesbite you will see an inglorious mess of 0800, 0845 and mis-statements about the costs of calls:

https://www.ukpowernetworks.co.uk/statement-of-services?dm_i=1T2Q,7Y66A,A3WU1I,W...

For example on one page it says in a footnote about an 0800 number: " Calls to these numbers will cost no more than a local rate number from a mobile. If you get inclusive minutes
in your mobile phone package, calls to these numbers are included.". These calls are always free.

There are 0845 numbers in palces where they are not allowed (probably now outdated) with no declaration of costs.

And this bit of inglorious nonsense: "We understand calling 0800 numbers from mobile phones or abroad is expensive so calls to these numbers will be charged at your standard network rate for calling landlines.
Calls to these numbers are normally included in any inclusive minutes you have on your mobile contract."