What can I use as a boot dip or spray?

You should use a disinfectant which has been approved by DEFRA, and tested to kill off the Avian Influenza virus, such as Nettex Poultry Virocur. The DEFRA Approved list is shown here http://disinfectants.defra.gov.uk/DisinfectantsExternal/Default.aspx?Module=ApprovalsList_SI and can be filtered at the top to show those whch apply to AI.