Some great work goes on around here and since it's most often a very lonely business trying to figure out where the goalposts have been moved to, I've had a great read so far.
I'll give you my situation right now (partly to try and understand it myself by writing it down) and would appreciate any feedback on it.
Apologies in advance for the head-twisting of the dual citizenship thing

I'm Belfast-born and therefore entitled to both UK and IRISH passports and citizenship. At the moment I've only ever had an British passport but for the purposes of applying for a residence card for my japanese 'durable partner' (4 years cohabiting in April), I understand that I'll be applying via the European method by using my Irish citizenship to be 'an EEA national currently outside country of origin' as is required.
For those in the know, what this means is that I haven't gone anywhere (I'm still in Belfast) but just changed the frame of reference as the dual citizenship allows. Therefore I'm acting as an Irish EEA National who is in the UK (N.Ireland) and using that Irish EEA National status to support my partner's residence card application. That way, I'm not a UK National for HO purposes.
Now...the thing is...I don't want to surrender my passport as I need it to get around for work (I'm self-employed) so I'm wondering if sending my previous cancelled passport together with a copy of my current one together with my birth certificate will satisfy the HO?
As I see it, I have to identify myself which the old passport does (and the copy of current one reinforces) AND confirm my place of birth to establish my Irish citizenship (the passports state place of birth anyway but I've been told that sending birth certificate is often required unless you're sending an Irish passport in my situation)
Any thoughts or experiences with this, anyone?
Thanks in advance....