Question

I want to know if the user is browsing from the UK, or using a UK browser. If not, I can safely assume that they are a US visitor.

I can lookup their IP in a database, but this is a bit slow and I’d rather just find out from their User-agent or some other HTTP header.

It doesn’t matter if it’s not always correct, and there are ambiguities (such as someone using a US computer in the UK).

Answer 1

You could probably check for Accept-Language header field.

14.4 Accept-Language

The Accept-Language request-header
field is similar to Accept, but
restricts the set of natural languages
that are preferred as a response to
the request. Language tags are defined
in section 3.10.

   Accept-Language = "Accept-Language" ":"
                     1#( language-range [ ";" "q" "=" qvalue ]

)
language-range = ( ( 1*8ALPHA *( “-” 1*8ALPHA ) ) | “*” )

Each language-range MAY be given an
associated quality value which
represents an estimate of the user’s
preference for the languages specified
by that range. The quality value
defaults to “q=1”. For example,

   Accept-Language: da, en-gb;q=0.8, en;q=0.7

would mean: “I prefer Danish, but will
accept British English and other types
of English.” A language-range matches
a language-tag if it exactly equals
the tag, or if it exactly equals a
prefix of the tag such that the first
tag character following the prefix is
“-“. The special range “*”, if present
in the Accept-Language field, matches
every tag not matched by any other
range present in the Accept-Language
field.

  Note: This use of a prefix matching rule does not imply that
  language tags are assigned to languages in such a way that it is
  always true that if a user understands a language with a certain
  tag, then this user will also understand all languages with tags
  for which this tag is a prefix. The prefix rule simply allows the
  use of prefix tags if this is the case.

The language quality factor assigned
to a language-tag by the
Accept-Language field is the quality
value of the longest language- range
in the field that matches the
language-tag. If no language- range in
the field matches the tag, the
language quality factor assigned is 0.
If no Accept-Language header is
present in the request, the server

SHOULD assume that all languages are
equally acceptable. If an
Accept-Language header is present,
then all languages which are assigned
a quality factor greater than 0 are
acceptable.

It might be contrary to the privacy
expectations of the user to send an
Accept-Language header with the
complete linguistic preferences of the
user in every request. For a
discussion of this issue, see section
15.1.4.

As intelligibility is highly dependent
on the individual user, it is
recommended that client applications
make the choice of linguistic
preference available to the user. If
the choice is not made available, then
the Accept-Language header field MUST
NOT be given in the request.

  Note: When making the choice of linguistic preference available to
  the user, we remind implementors of  the fact that users are not
  familiar with the details of language matching as described above,
  and should provide appropriate guidance. As an example, users
  might assume that on selecting "en-gb", they will be served any
  kind of English document if British English is not available. A
  user agent might suggest in such a case to add "en" to get the
  best matching behavior.