Incoming badge packet?

[kingsley]

For 20 years or more I have been selling or trading the odd badge from Australia to UK using small padded envelopes with a Customs Declaration sticker. The cost has been AUD$3.20 for Economy Air but Australia Post has used Covid 19 as an excuse to abolish this service and charge $25 for Airmail Parcel Post. This is too much for a $5 badge but I can still send documents by Economy Air in an identical sized envelope.
What would UK Customs do if I sent you a padded envelope with a badge inside and no customs declaration?