Dear Khan -
As you know, application for a visa to visit the UK from Pakistan takes
longer than the time left between now and the conference. This is
unfortunate as we would have gladly provided you with the necessary formal
letter of invitation had you contacted us when the conference was first
announced, several weeks ago. I'm sorry it's not going to work out for you
this time.
If you're planning to attend Wikimania 2014, which is taking place in
London in August next year, I suggest that you get in touch with the
organisers about support for your visa application as soon as possible.
Further details about Wikimania London 2014 available at
http://wikimania2014.wikimedia.org
Best wishes,
Toni