Bitstamp has all of its operations in Slovenia and as far as I can see never needed to incorporate in the UK.
I heard that they wish to move operations to the UK hence maybe re-animating the Ltd.
They may have some operations in Slovenia, but their website clearly states their operating as UK company. In "about us section":
About Us
Bitstamp Ltd.
5 Jupiter House
Calleva Park, Aldermaston
Reading
Berkshire RG7 8NN
United Kingdom
From the T&C:
"As used in this Terms of Use, Bitstamp" refers to the company
Bitstamp Limited, including, without limitation, its owners, directors, investors, employees or other related parties. Depending upon the context, "Bitstamp" may also refer to the services, products, website, content or other materials (collectively, "Bitstamp Services") provided by Bitstamp."