There are currently 16 species of bat known from the UK, six of which are considered, conservationally-speaking, “Vulnerable” (Vu); four are “Rare” (R); two are “Endangered” (En); and only four are not threatened to a sufficient extent to warrant adding to a conservation list. The 16 UK species and their conservational status are as follows (S = Scotland; W = Wales; I = Ireland; E = England):
Horseshoe bats
Greater horseshoe bat (Rhinolophus ferrumequinum) (W, E, En)
Lesser horseshoe bat (Rhinolophus hipposideros) (W, I, En)
Pipistrelle bats
Common pipistrelle (Pipistrellus pipistrellus) (S, W, I, E)
Nathusius’ pipistrelle (Pipistrellus nathusii) (W, I, E, R)
Pygmy pipistrelle (Pipistrellus pygmaeus) (S, W, I, E)
House bats (Serotines)
Serotine (Eptesicus serotinus) (W, E, Vu)
Noctules
Leisler’s bat (Nyctalus leisleri) (S, I, E, Vu)
Noctule (Nyctalus noctula) (S, W, E, Vu)
Little Brown bats
Bechstein’s bat (Myotis bechsteini) (W, E, R)
Brandt’s bat (Myotis brandtii) (S, W, E, Vu)
Daubenton’s bat (Myotis daubentoni) (S, I, W, E)
Natterer’s bat (Myotis nattereri) (S, I, W, E, Vu)
Whiskered bat (Myotis mystacinus) (S, I, W, E, Vu)
Barbastelles
Barbastelle (Barbastella barbastellus) (W, E, R)
Long-eared bats
Brown long-eared bat (Plecotus auritus) (S, I, W, E)
Grey long-eared bat (Plecotus austriacus) (E, R)
Up until the January 1990, when it was declared extinct in Britain, the mouse-eared bat (Myotis myotis) was the 17th member of this list. Since its removal, there have been occasional sightings of hibernating Myotis myotis in the UK but they've been of single individuals and I'm not aware of any evidence to suggest that the species has begun recolonising the British Isles. Additionally, the regions above should be considered tentative. In Wales, for example, there are only isolated records of P. nauthusii, E. serotinus, and M. bechsteini. Similarly, species identification is easier for some bats than others, and some species are very difficult to tell apart. M. brandtii vs. M. mystacinus or P. pipistrellus vs. P. pygmaeus, for example, are often recorded as simply "brandt/whiskered" and "pipistrelle", respectively.