Hitting that 14-15% gradient with no graduation is causing pretty severe tire slippage; even with Computrainer clamped down fairly tight (calibrated to around 3.0). Nothing to do with effort or being hard, it’s just such an extremely short “hill” to be that steep. I’ll probably get some negative votes on this one 
, but my vote would be to lessen the severity of the incline just a bit.
I use the Conti Trainer Tire on my CT with no slippage. Try speeding up a bit (lot?) right before it.
Also, have you cleaned the tire with isopropyl alcohol?
You can of course turn down the hills overall by reducing the trainer effect or difficulty.
Speeding up to high cadence helps, and I thought about adjusting trainer difficulty (or cassette selector, as I call it…). But that’s kind of the point, I don’t mind worrying about that stuff when I’m going to be tackling the big hills/mountains; just seems like this is an unnecessary blip when riding the London route. Certainly not a big worry, I’ll carry on with my training and it will soon be forgotten
I’d leave the trainer level where it’s at, as it would be a “global” adjustment affecting the entire game across all ride locations. Speed up is my recco. In the “real” world if you saw a steep like that coming you’d up your cadence or accelerate into it to get over it. Clean the tire up or if possible use a trainer tire. I’m calibrated to 2.1 with no slippage.