The length of the mite is 0.4 mm. Host is Tandonia budapestensis in the first and Deroceras reticulatum for the other two pictures.
So what does the mite do to the slug? It looks rather small to cause much damage.
When one individual slug is infested with hundreds of mites there could be an adverse effect.
"Knowledge of the anatomy and contents of the alimentary canal of R. limacum was obtained by using transverse and longitudinal sections of adult mites. Histological and histochemical techniques revealed the absence of slug mucus and the presence of slug amoebocytes in the gut caeca of the mite. The evidence available supports the contention that the slug mite is a blood feeder and obtains this food by producing a feeding tube or ‘stylostome’ in the tissues of its host."
Lat/Lng: 52.217396666667, 0.118275
OS grid ref: TL448598