As far as I am aware there are no train services in the UK where seat reservations are compulsory.

Advance booking cheapie tickets come with seat reservations as part of the package, but the number of passengers travelling on such tickets is relatively small, and people who travel on full-fare tickets rarely bother with reservations.

So, as the previous poster said, it is effectively free seating. If you don't like your allocated seat just sit somewhere else - just make sure you don't pick a seat that is marked as reserved.