Train from London

You can book online. I used the Scotrail site, but I think you can use Britrail. Nigel or Mark can likely say for sure. What I did was pick it up at the station. Make sure grAb the ticket and the receipt from the ticket machine. I am pretty sure that you need to have the credit card with you that you used to buy the ticket.

In terms of making sure that you are on the same train, check times and departure station and train number and date! I speak sadly from experience decades ago when some friends and messed up on a trip to Athens.

Pam

They don't use train numbers in the UK. All trains are referenced by their departure time.
Does the England pass cover a trip to Edinburgh? If not, you may need to buy a separate ticket from Berwick-upon-Tweed to Edinburgh.

I had the same question as Chris whether you got the BritRail pass for the whole island of the smaller BritRail England version. Seat reservations are optional and free for pass holders to make at train stations up to a day ahead; be prepared to tell your friend's car number and seat number if you're requesting one nearby. Otherwise, you can simply to board the same train with your friend and take any available seat.

I guess I have been riding too much Amtrack! I just know that when you are coordinating you really should double check departure and arrival times. Been there and not done that! ;(

Pam