What is this rodent?

I would also say mouse.

I'm not an expert and I think small rodents can be difficult, especially as the face isn't shown that well in the photo, but I certainly don't think it's a rat.
I agree the ears are very evident, but I'm not convinced they are big enough for a Wood Mouse and unless it has lost most of its tail, it's not a mouse at all. I also think the eyes are not large and prominent enough so, for me, it's probably one of the voles. Judging mainly from the tail length it may well be a Field Vole.