I have been using Primus (now Fuel Broadband) for some years (for land line, not broadband) and have had no problems using 18185 with them.  Their line rental has gone up and I am in the course of switching my landline rental to the Post Office who I thought allowed 18185.  I have now had a phone call from the Post Office support (in response to a web query sent 3 weeks ago) saying I can not use 18185.  Has anyone direct experience of using 18185 with the Post Office (not via the 0800 route)?  I note the T&C ban pre carrier selection but make no reference to override providers.

It's most likely that whoever wrote the P.O. T&Cs thought the likes of 18185 were CPS because they did not understand the terminology and did not appreciate the difference between CPS (which uses a permanent override code by way of programming of the exchange) and 18185's (the subscriber's) use of an override code on a per call basis.

AFAIK, those with P.O. line rental (ISTR that's now Talk Talk's) cannot use 18185.

I would "think" that, logically, it's as Heinz says: Post Office Home Phone is now provided by TalkTalk and therefore the 18185 prefix is not available, the service only being available via its 0808 or 020 number with different call rates.