As I am sure most people on here know, trial lessons run very close to the line in most cases (most being obvious pleasure flights in reality) but the CAA is not going to do anything about them because to do so would destroy the PPL flight training business, most of which desperately needs the income from trial lessons.
The problem arises because, in the UK, almost any regular aerial work needs an AOC, but an AOC (which is a great moneyspinner for the CAA) is so expensive to get and to keep - of the order of £20,000 as a baseline - that the "trial lesson" exemption had to be created to enable PPL flight training to stay in business.