Bulgaria post registered mail to United Kingdom (UK) returned

Parcels / package was posted via Bulgaria post registered mail to the UK on the 19 Jun 2017 and arrived in the UK on the 13 August which took around 24 days and then it eventually returned to the sender without even making registration to the UK post office (Royal Mail or Parcel Force). This could be due customs problem. Below is tracking info from Bulgaria post

ITEM n° RI232782879BG new search
Local Date and Time Country Location Event Type Extra Information
19 Jun 2017 16:14:38 BULGARIA SOFIA LC/AO Receive item at office of exchange (Otb) Получаване на Изх.пратка в Межд.разменна станция (В 09)
19 Jun 2017 17:58:34 BULGARIA SOFIA LC/AO Insert item into bag (Otb) and Send item abroad Влагане на пратка в чувал (C 09) и изпращане в чужбина
13 Aug 2017 04:30:00 UNITED KINGDOM LANGLEY HWDC Send item abroad (EDI-received) Изпращане на Изх.пратка в чужбина
14 Aug 2017 07:19:11 BULGARIA SOFIA LC/AO Receive item at office of exchange (Inb) Получаване на Вх.пратка в Межд.разменна станция
14 Aug 2017 10:14:18 BULGARIA SOFIA LC/AO Send item to domestic location (Inb) Изпращане на Вх.пратка в РСЦ/ОВ
14 Aug 2017 13:37:00 БЪЛГАРИЯ 1900 БРСЦ СОФИЯ Пратката е приета
15 Aug 2017 07:21:00 БЪЛГАРИЯ 1504 СОФИЯ 4 Обработка на пратка
15 Aug 2017 11:41:00 БЪЛГАРИЯ 1504 СОФИЯ 4 Първо посещение
15 Aug 2017 11:42:00 БЪЛГАРИЯ 1504 СОФИЯ 4 ГИШЕ