The Post Office has started a new Phone Service called Home Phone.  
They say:

“Looking for a better value phone service?  Then switch to NEW Post Office HomePhone now!”

The day rate is 2.5p/min for all local or national calls.

They charge 4p/min for 0845 numbers and 10p/min for 0870 numbers.

I followed this up by ringing The Post Office, asking them how they could justify 10p / min for 0870 and what do they charge for 0871 as it is not listed. They had no idea on the 0871, said that 10p was the standard rate for 0870. Where is OFCOM??? This is a new company and they cant even get it right, I dont hold much hope of OFCOM coming down on the side of the consumer. It would be interetsing now to use the FOI Act to ask why Oftel let the companies get away with this and why Ofcom has continued to allow it,
Keep asking the questions,
>:(

Of course, Ofcom will tell you that it's up to the "commercial discretion" of the Post Office as to their charges. So long as people take up the service, Ofcom is happy because it is introducing more 'competition' into the 'market'.

I have just posted a reply on the Radio 4 discussion area, but will repeat it here as it will probably work for any other call provider, such as the Post Office?

"There is a way to defeat Onetel's cost of 0870 calls. If you have signed up to call automatically (carrier pre-selection) via Onetel, but still pay your line rental to BT, you can 'revert' to BT for a specific call by dialling the prefix 1280 before the call. This will route the call via BT and so you will be charged by BT - not Onetel. This means you can at least take advantage (if that is the right term!   ) of the cheaper rates that BT charges at various times of the day.  I did this all the time when I lived in the UK, and the 0870 charges then appeared on my BT account. Obviously, wherever possible, I chose to make the calls at a time when the cost was the least!

The advantage of this is that BT have their charges regulated - the other telcos don't. It's still an incredible rip-off, but at least it is one way to partially offset the cost."

Comments on this welcome (even though - thankfully!! - it doesn't affect me any longer. Most - 95%? - US companies have a 1-800 = 'freecall' number.......   ;D  )

According to the POHP's website, the charges to 0870 numbers have been reduced to that of those on BT Together. 0845 still remain higher than BT Together on weekdays being 3.3p/min in the daytime and 1.1p/min in the evening. Weekend rates are the same. Daytime is 08:00 to 18:00 with PO whereas BT's is 06:00 to 18:00.