IATA - What to Know About How to Ship Lithium Batteries?
MacSpeedy's - Lithium Batteries by themselves or power banks are dangerous goods and are not allowed for shipment from the US or UK. These batteries are allowed within devices such as phones
Account prohibitions and restrictions | Parcelforce Worldwide
LITHIUM BATTERY WARNING Caution Label Shipping Postage Royal Mail 100 200 £5.99 - PicClick UK