why the huge increase in price for train tickets from London to York?

I am looking at an 8:30 departure on Friday the 24th of February for just under 15 pounds. Are you pricing one specific day and getting that 76-110 pound price? I did actually run into this very issue on our trip last summer. I had been pricing Advance tickets for numerous runs and prices were consistent for weeks. But when I went to make my actual purchases, the London-Bath route I needed had gone up by nearly 70 pounds per ticket. I posted this very question to the helpline and the only real explanation that made sense was that I was buying tickets for that route on the very same weekend that thousands of people were headed out towards Bath for the Glastonbury music festival. Given that I could not bring the price down and I could not change my travel days, I ended up buying 3-day RailPasses and it ended up being a fantastic decision. We could travel any 3 days, did not have to commit to specific train times, and never had trouble finding seats. I would check around a bit more on the pricing. If you're really stuck with a price that high, a RailPass might work for you if you're traveling around a bit. The low season pass is $185 for travel any 3 days. I think regular season is about $229.

Thank you for your reply. I am looking at a specific day and like you last summer, I've been watching the prices for several weeks in anticipation of the day when I can get in and purchase our tickets. But now this week they've taken that sudden huge jump in price. Unfortunately the rail pass won't work for us because we'll ony be in England for a long week-end and the trip to York is the only time we'll be using the train. I just emailed the B & B where we'll be staying to ask if she knows if there is something special going on in York that week-end.
Thanks again for your help.

There are more than just the route from Kings Cross to York, operated by other TOCs (Train Operating Company). Have you looked at other routes or times? What day of the week?

We know you are trying a specific day, but we don't know which one. I looked at the 28th and found singles for as little as £10.45. Dep Arr Duration Price 08:00 09:50 01:50hrs £25.95 08:30 10:32 02:02hrs £10.45 08:35 10:45 02:10hrs £25.95 09:00 10:51 01:51hrs £25.95 09:08 11:31 02:23hrs £10.45 09:30 11:32 02:02hrs £25.95 09:35 11:46 02:11hrs £25.95 10:00 11:51 01:51hrs £40.05
10:30 12:32 02:02hrs £13.00 for example

The 8:30 on Friday the 24th is at £14.60.