Durham Tees Valley Airport was not included in the survey
DTV accounts for less than 0.075% of commercial passenger air transport in the UK, or 1 passenger in every 1,350. Newquay and Derry both have more passengers than DTV but are missing from the survey as well.
While Which? has a desire to be objective, they also need to make editorial judgments as to relevance - no point waffling on about a product that very few of its readers will even consider purchasing or using. Should Which have included Dundee and Gloucestershire airports as well ? What about all the airports on the Scottish islands ? You have to draw the line somewhere...
Furthermore, when carrying out a consumer survey, there is also the need to ensure that the data is statistically reliable - i.e. you have enough responses about an airport to ensure your summary data is not unduly swayed by the opinion of 1 single person. If DTV accounted for 0.075% of the 11,500 replies Which? received, then about 9 of those responses would be about DTV - generally within statistics you want to see at least 25 responses before you can draw any meaningful and reliable conclusions. It is likely that Which? decided not to include DTV as to do so would have meant publishing unreliable and biased opinions.