Intoelectrics... agree, I'm not a sparky, but feel that RCBO's can fit into design alongside RCD protected circuits for discrimination as you stated, especially the kitchen. Also agree with calling in an electrician to assess my install and signing off before energising system. Doing this for the enjoyment and knowledge...but safety is paramount!
As you're in Cardiff, you cannot use the third party certification route. In Wales we are required to follow the 2010 version of Approved Document P which (a) makes the kitchen a special location requiring any work involving changing the fixed wiring a notifiable job and (b) did not contain the provision for Third Party Certification.
This provision was introduced into Approved Document P in 2013 which is after the Welsh Assembly took control of the building regulations (which is why we're still working to the 2010 version).