Technically that could happen. But if you allocate seating properly, you'll just allocate a London to Wakefield passenger into that seat instead. That's why seat selection needs to be chargeable.
Yes charge for the right to be able to change your seat.
A Window seat rather than the default aisle seat (or windowless seat), that will cost you an extra £5
Want that table seat so you can carry out work, or booking 4 seats and wanting to sit together? That will be an extra £10 each please. Don't want to pay?, then one member of your group will end up in coach A, another in coach B, but we will be kind to put 2 people in coach D, but at opposite ends. Don't you dare move seats as the "Seat Reservation guard" will charge you a penalty of £100 each. This is for your own safety, we don't care that the coach you are sat in is full and the other coaches are empty.
Don't want to pay, your seat reservation will remain a secret until you check into the train and you will get what is left. Fail to check in and that will be a penalty of £100. Board one of our lovely trains without a seat reservation, yep that will cost you £100.
Meanwhile in the real world, passengers will simply flock to the plane or car rather than paying over the odds to pick a decent seat for the 4 hour journey from London to Edinburgh