UK and EU law state that products sold in the UK must be advertised in metric units (Wikipedia). However, Subway's sells sandwiches that are "footlong" or "six inches", and at least two pizzeras I have been also advertise their pizzas by diameter in inches (online example). Considering that traders have been prosecuted for not using the metric system, how can Subway's and pizzerias legally do so?
From the Wikipedia you linked:
Goods and services sold by a description are not covered by weights and measures legislation; thus, a fence panel sold as "6 foot by 6 foot" is legal, as is a 6 x 4 inch photograph frame, but a pole sold with the price described as "50 pence per linear foot", with no accompanying metric price, would be illegal.
In both cases the measurement is a description of the product, not a unit by which the consumer is charged.