It would be fair to say that if the primary purpose of your trip is take the images and write and article then the cost of the trip would be tax deductible, assuming that your visit is limited to the time you are working.
For example the costs of a short visit to take pictures for a holiday brochure would be OK as long as the visit is limited to working days.
However, the whole issue becomes more problematic if the trip is extended to become a holiday or you take your spouse/partner with you.
The HM Revenue & Customs guidance on this is clear in that if the purpose of the trip is to have a holiday with an element of work involvement then the costs would not be tax deductible. Indeed, there has been a test case on this very issue involving a solicitor who took his wife on holiday to America & Canada, but whilst there he also attended meeting of the American Bar Association. He tried to claim any costs attributed to himself only, however HMRC argued successfully that "..... the purpose of Taylor's visit was not exclusively to enable him to attend the conferences but substantially included holiday and social purposes"
It was essentially on these grounds that the costs were not regarded as tax deductible
I am looking at attending the SWPP conference in January 2008, is this a business expense?
Yes - assuming the primary purpose of your visit to London is to attend the conference. The allowable costs would include, travel, hotel, subsistence (but not alcohol - sorry!) seminar costs, etc. However, if you were to take your spouse/partner then any costs relating to their stay would not be regarded as a business expense.
I am afraid that if you were to extend your trip to include a few days sight seeing then HMRC might want to argue that the trip was not 'wholly and exclusively' relating to your business and want to omit some or all of the expenses.
The Societies of Photographers Convention and Trade Show at The Hilton London Metropole Hotel ...
You have 256 days until The Societies of Photographers Convention starting on Wednesday 11th January 2017