Lots of issues here.
A good guide to recording legality can be found here...
If you have, or are prepared to install, a voice modem an excellent product here...
I think you are right that for a party to be able to use a recording in a court of law then notice must be given. From experience you are right that call handlers in commercial organisations sometimes decline to continue a call if notified. I have no idea if that is official policy or not....
I think you are also right about the propensity to "fail" to record when the content doesn't suit.
On balance I think its better for the customer not to give notice because the likelyhood of ever going to court is remote. For example Ombudsmen are not courts of law. It's better for a customer to have a detailed explanation of an issue, informed by a recording, than no detailed explanation. Turn the tables. If an organisation's call handler won't continue under notice, record that response. Record future calls without notice. If the organisation ever denies it declines to continue under notice you have the proof that you were lied to.
If I have a serious issue with an organisation I would always invest £10 in a Subject Access Request requiring the provison of all data held, including customer notes and call recordings. This is an official way of finding out exactly what is available and costs the organisation much more than £10 to fulfill.
Another thing you don't mention - the number of times an organisation claims to have posted an important letter which never arrives, statistically well in excess of claimed Royal Mail loss rates. Hard to prove, but on balance don't think it's a Royal Mail issue.