Solution 3 has been proposed by customer service in the past.

Thanks

Just been to station for reservations to London Euston and ticket office staff suggested the same.

Chris

Hello, I appreciate that this information is likely too late to be useful, but it might be worth mentioning regardless that our customer service team can simply update your passport number for you. If we are not able to respond to you immediately, due to our delayed response time, then please travel with both passports until we've managed to update your passport number. An exchange is not necessary.