ESW (100% owned by Asendia) and DPD both have long standing capability to allow UK based ecommerce retailers to effectively ship internationally.
The UK is an especially cost effective place to ship from, in terms of parcel costs and efficiency including customs handling and ability to set up bonded logistics to mitigate duty/VAT payments, although warehouse land and labour costs are higher than many places in the world.
In turn, Asendia is a joint venture of French Post and Swiss Post, and is a popular choice for international parcel carriage (competing with International Distributions Service’s–better known as Royal Mail UK–GLS divisions in various countries including US)