broadgage: 
 

Diesel fuel can be cheaper than mains electricity, and has been on a few days when spot electricity prices reached record levels.

I would not expect that grid electricity would REMAIN more expensive than diesel fuel for any extended time.

Do British trains run on grid electricity? Or do they have their own generating plant?

Almost all transport operators hedge their fuel costs. Airlines do it years in advance. Why wouldn't train operators do it? I can't see how you could possibly run a rail freight operation without hedging your energy costs. 

broadgage: 
 

Diesel fuel can be cheaper than mains electricity, and has been on a few days when spot electricity prices reached record levels.

I would not expect that grid electricity would REMAIN more expensive than diesel fuel for any extended time.

Do British trains run on grid electricity? Or do they have their own generating plant?

Almost all transport operators hedge their fuel costs. Airlines do it years in advance. Why wouldn't train operators do it? I can't see how you could possibly run a rail freight operation without hedging your energy costs.