You’ll probably get the difference between first and standard refunded.
Chapter and verse from the NRCOT:
31.1 If you have a frst class Ticket and the train service you use is shown as offering frst class accommodation at
www.nationalrail.co.uk, but when you travel frst class accommodation is not provided or is otherwise fully occupied, you may claim a refund. The minimum refund to which you will be entitled will be the difference between the price of the frst class Ticket purchased and the cheapest valid standard class fare available on the service you used.
Assuming you held a First Off Peak Single GC ONLY then you can expect the difference between that and the Standard Off Peak Single GC ONLY to be refunded.
I’m assuming your ticket was to Zone 1 London and cost £93.80. The relevant standard class fare is £66.20, making the refund due to you £27.60.
You may get more if there is an exacerbating factor like you having reserved a seat, but for Christmas travel, if you didn’t have a seat reservation, I think the minimum is all you’ll get.