Smoke generators are usually used for model railroads.  Please have
a look at the famous German product LGB (www.lgb.com).  This company
has a vendor for Great Britain:

Grossbritannien:

  The Hobby Company Limited
  Unit B - Chiltern Trading Estate
  Grovebury Road
  Leighton Buzzard - Bedfordshire LU7 4TU
  Tel: 0044 1525 385798
  Fax: 0044 1525 385778
  enquiries@hobbyco.net

They have smoke generators for 5, 18 and 24 volts.  Select "Products"
from the lower end of screen, then enter "smoke" in the field "Search
with words" and click "Go".  In the displayed table are found some
smoke generators.

Peter Leutwiler
Switzerland