Would anyone know if the UK Customs or the Royal Mail even on second instance are required to inquire with the recipient of an incoming  postal parcel's content value were either the CN22 or CN23 Custom Declaration Form is incomplete, incoherent, damaged or missing ? Or not at all. So that they will simply send it back to it's sender without seeking any further after having spent 3 weeks with them.

Am based in Belgium and here the Belgium Customs do inquire with the recipient when needed. It even shows on the Belgium Post tracking internet site and the recipient also gets a regular letter in the mailbox to comply with it. Allowing the recipient to provide them with the informations they need to proceed with the import procedure before sending it back to the sender if ever they can't proceed it as such. 

I have never known Royal Mail ask or make any enquiries as to what the value of the package is, should the customs declaration be missing or unreadable. Sometimes if the value is clearly shown, they leave a card telling you that you need to pay X amount and collect the parcel from the sorting office. However, I have had many packages delivered from the US where the value is clearly stated, no import duty or VAT has been paid, and the parcel has been delivered as normal without any further payment required. From the posties I know they see it as a pain for them to check import packages, but the government require it. But it seems from my experience anyway that most just go through without any checks as they don't have the time to do this, let alone chase you on whats in the package etc. 

Thanks for that. So the UK Import Customs Duties have no other mandatory requirements than to either cash in due import duty taxes if any or to send the parcel back without investigating further.

I would have expected the UK Customs to do (like any other) like it was before the internet days to send the recipient a letter asking to bring forth proof of the purchase if any or to be accounted as gift.

Now I know as I just have my first mailer that's on its way back being returned to me. Where before they all went through easy peasy or almost. But never returned to me. Until now. Hence my quest.

The Belgium Post has become such a terrible administration it was my last resort to send them through like that or else the mandatory shipping price, huge packaging and pre-formatted CN23 format a nightmare to deal with.

Now hopefully I've been advised that the UPS services are reliable, cheaper and efficient enough to avoid the Belgium Post hassle and their unforgiving CN23 mandatory Custom declaration form and packaging. I'll try that.