Whilst the great joy of traveling with a Britain Pass is to turn up and go, and seat reservations are optional, they are recommended in certain instances if you want to be guaranteed a seat on a specific train and time, for example:
In these cases a Britain Seat Reservation is a good option. Rail Pass holders can make a seat reservation for the route of their choice on nearly all inter-city train routes throughout Britain, per person, per train for a nominal fee.
A seat reservation is only valid for use on a specific train, traveling on a specific date and time indicated on the seat reservation. Available in Adult: First or Standard class but must be the same class type as the Rail Pass. If traveling with a Rail Pass in Standard class, please note that you are not eligible to travel with a seat reservation in First class.
Simply print out the itinerary and bring it on board to show train staff the NRS Booking Reference # along with your Rail Pass.
Britain Seat Reservations are non-refundable.