You'll be used to this response... but please sit down before reading further!
1.5kg, 50cm x 25cm x 15cm is about as small as I can make it in a box. I can probably just wrap in bubblewrap and then wrap in plastic if that's OK with you...
Parcel2Go: cheapest £63.40
ROyal Mail airmail: £20.70, delivering in 3-5 days. To add a signature on delivery would be £5.40 extra.
I need to check that this would meet definitions of a "small packet", but that doesn't seem too bad compared to a courier!