There are no different seats for G, N, Q, U, L, V, T, H, K etc. All these fare classes are for the same Economy seats. They just differ in price, the number of miles you earn, ticket restrictions and availability.
As these fares are all for Economy, customers typically buy the cheapest fare class available at the time of booking.

If you insist on paying more (and flying with the ticket conditions of a higher fare class), you can always call them and make the booking in K class, for example, even though (cheaper) L class is still available.