These three options are on the royal mail site, check VERY carefully which one you chose.
Sometimes you can make an error and not see it repeatedly when back tracking.
"International Tracked & Signed
Tracked & Signed is a combination service offering full end-to-end tracking, signature on delivery and an online delivery confirmation.
International Tracked
Ideal for sending important or valuable goods, offering tracking at five points after despatch as well as an online confirmation of delivery.
International Signed
Your item will be tracked until it leaves the UK and only handed over at its final destination once we have a signature from the recipient."
did you get as far as this page?
"Enter your reference number
Enter your reference number
Your tracking code is between 9-21 characters and may be called a reference number, a barcode or simply a reference. Please do not add any spaces or hyphens (-).
Enter your reference number
E.g. AB586985609GB
Track item
Find your reference number
Special Delivery Guaranteed
Post Office® Receipt
Learn where to find your tracking number on your label, Post Office receipt or Something for you card.
You’ll find your reference number on your tracking label or Post Office® receipt.
More on finding reference numbers."
You said you did not get a receipt, did you get a tracking label?