According to Amazon, the major changes to the UK webstore product include (reported verbatim):
UK re-launch: Overview
We are very pleased to announce the re-launch of Amazon Webstore in the UK using Amazon Payments Europe for all payment handling. This has allowed us to launch two very important large seller features for our UK sellers, namely seller merchandisable checkout and tiered pricing.
Seller Merchandisable checkout
In addition to allowing sellers to merchandise their checkout pages, the introduction feature also gives sellers enhanced customer recognition features and additional APIs (e.g. account creation).
Tiered pricing is another important feature: it allows us to both offer competitive fees to our larger sellers, and to incentivize existing sellers to channel more sales through their Webstore.