Easy,
I know next to nothing aboutthe airspace around Bristol and Cardiff, but here's a basic view:
You usually talk to Bristol, then Cardiff, then London...correct?
Now, for you to go straight from talking to Bristol to talking to London, Bristol ATCO has to phone Cardiff ATCO and ask for permission to go straight to London, and if there is anything to affect. In the meantime, what happens if London want you at a different level?.....They know that they usually get you from Cardiff, so the London ATCO talks to Cardiff, who he assumes you're talking to, to co-ordinate, but Cardiff says that, no, Bristol are still speaking to you. So London ATCO phones Bristol ATCO to co-ordiante.
ATC standing agreements are based on the fact that each ATCO knows to whom all the a/c in neighbouring airspace are talking to, in case he or she needs to co-ordniate.
Three or more phone calls and three-way co-ordination can all get a bit messy when the ATCO is busy (not neccessarily co-incident with the frequency being busy).