1. We aren't Royal Mail.
2. The carrier at the UK end was Parcelforce.
3. Heathrow was the last place that An Post could track the items to, not the delivery address. They were not detained at Heathrow, they passed through pretty quickly.
4. Parcelforce marked the items as delivered on 29/08, to a location served by their Milton Keynes depot (for more info, try your tracking numbers at https://www.parcelforce.com/track-trace
5. If you disagree that the items have been correctly delivered, you need to claim via An Post.