I just purchased a Britrail Flexi Pass for travel this coming September. My itinerary is as follows:

Sept.17..London to Edinburgh (Plan on taking the 9:00 am train)
Sept. 21...Edinburgh to Rannoch via GQS
Sept. 23rd...Rannoch to Oban
Sept. 26th Oban to Edinburgh via GQS (arrive GQS @ 11:28, depart @11:45 arrive Waverley @ 12:35 and want to make the 1:00 pm train to Kings Cross. Yes,I know, a long day.

I have traveled with and without reservations and prefer to have a reservation if trains are going to be crowded. Since my plans include traveling both to and from London and Edinburgh and to Edinburgh from Glasgow on a Friday, I thought trains might be crowded and a reservation would make for a little less hassle. Also the Edinburgh Rannoch/Oban/Glasgow route is a popular one and although I've done it before, it's been several years and can't remember how crowded it was.

I can get reservations for Britrail for $12 which I think is a bit high. Would it be cheaper to wait until I get to London and purchase them at one of the train stations a couple of days prior to departure?

Any suggestions or advice would be welcome. Thanks

I believe that seat reservations for these trains are free if bought at a station.
Even on a Friday, trains are generally busy only in the afternoon and evening, when people go away for the weekend.
In Scotland, the peak holiday season is earlier than in England, and late September is much quieter.
I don't think you'll have any problem booking seats when you get to the UK.

You only need reservations for the London to Edinburgh and return. You can do that directly at the station, free. If possible you can do that the day before the trip up to Scotland or even that morning. Rail Europe fee is a rip off.

Thanks Geoff and Roger, just the kind of responses I was hoping for.

On Sep 26, it would be slightly easier to catch the 12:49 Virgin train from Glagow Central, arriving London Euston at 17:17. An advantage of the Virgin train is it only makes two stops on the way, so you get less disruption with people getting off and on the train.

Roger suggests you can make your seat reservation the day of your journey. These have to be made at least 2 hours in advance of your train departure. You may not want to be at King's Cross at 6:30 am, queuing up to get your reservation for your 9 am train.

Thanks for the heads up on reserving seats. I will be at the British Library a couple of days prior to departing for Edinburgh, so thought I'd get reservations then and check out Kings Cross Station as it's been several years since I've been there.

I did look into the Virgin train from Glasgow and don't know why I discarded the idea. I'll have another look.