Are you self-employed? Then as it turns out there may be such a thing as a tax-free lunch.
As general rule if the costs of travel are allowable then modest subsistence (lunches, snacks etc) associated with such travel is an allowable expense. Itinerant traders (those that travel from one location to several other locations to perform their work) are the most likely to have such claims allowed. Sub-contractors working for a single client, at a single site may be denied subsistence expenses.
In either case get in-touch for advice specific to your circumstances.
Source info: HMRC BIM 37670
Enjoy saving tax?