Wholly and exclusively: expenditure having an intrinsic duality of purpose: food and drink
S34 Income Tax (Trading and Other Income) Act 2005
Carpenter eats to live
The case of Caillebotte v Quinn  50 TC 222 is described at BIM37660.
Where food and drink is consumed for the purpose of sustenance there will be an inevitable private purpose to the expenditure. Lord Templeman explained the taxpayer’s basic difficulty with the legislation and gave a prosaic example. A self-employed taxpayer, like any other taxpayer, must eat to live. No part of the cost of Quinn’s lunch was laid out wholly and exclusively for the purposes of his trade as a carpenter.
You should therefore disallow the costs of ordinary meals that serve the function of sustaining the taxpayer. It is immaterial that the physical demands of the taxpayer’s occupation require a greater consumption of food or that the location of the work place imposes a greater cost.