My site has carried on growing this year, but UK traffic has been virtually static, although it remains my largest single source.
I can monetise UK traffic better, so this is not ideal (a lot better than not growing!).
The site is a .co.uk, it has a reasonable number of links from British sites (but a lot of the strongest links are from the US), and the lang attribute of the root element is en-GB
What more can I do? Do I need to send the language in the http headers as well as in the document? Does the fact that the site is hosted in Germany matter? Does the address to which the domain name is registered (not in the UK) matter?