I've based my identification on colour. It is a lovely rich reddish brown which is characteristic of the adult bank vole, along with the contrasting grey underbelly.
The shape of the ears fits with the bank vole, they are rounded, not too big and held back away from the face (rather than held pointing slightly upwards as in the mice).
It is true we cannot see the tail, which in the bank vole is quite hairy and shorter than the relatively hairless tails of the mice.
What you have seen cold be a wood mouse. Wood mice have more pointy noses and a white underbelly, again they have quite long tails.
I recommend you try the FSC Key to British Land Mammals which is a handy leaflet to take out and about with you, containing all of the UK's land mammals.
Identification and Advisory Service