The mapping/accounts page is here.

Species-accounts are those from A Flora of Cumbria (1997), for species which are covered therein. Updating comments may be added in due course.
Stace (Ed. 4, 2019) describes, and keys out, ninety-three (!) species of Cotoneaster in Britain and Ireland. The Plant Crib has a useful key (1997; ‘minor update 2012’) by J. Fryer to the ‘twenty most likely species to be found in Britain’.
Tim Rich’s recent publication Identification Guide to British Schedule 9 Cotoneasters is available at cost in Kindle format. It covers the five
‘Schedule 9’ species (invasive plants under the Wildlife and Countryside Act 1981), bullatus, simonsii, horizontalis, integrifolius and microphyllus), with notes separating them from other closely related species.

Twelve species are mentioned in A Flora of Cumbria. The current county list stands at twenty-eight species and two hybrids… a somewhat alarming increase in twenty-seven years!

A Flora of Cumbria (1997) introductory account and key

Although there is only a single record in the early Floras, this genus is now known to be represented in the county by at least twelve species. Probably all but one have appeared during the last 50 years, becoming naturalised from garden throw-outs or by bird-dispersal from gardens, although plantings, especially in the Arnside area, cannot be excluded. With the exception of the three commonest species, Cotoneaster horizontalis, C. integrifolius and C. simonsii, all records have been checked by J. Fryer.
The following key is based on information which she has provided. [NB. A further species, C. rotundifolius Wallich ex Lindley, not keyed below, is listed in the Addenda page with the following information: Found by the roadside south of Uldale in 1995 (22.54, MP, 1995, det. J. Fryer). This species differs from Cotoneaster microphyllus in being more erect (-2 m) and having generally longer leaves (-2 cm) and larger flowers (1-1.3 cm across), which are always solitary.]

1. Lvs 0.5-1.5 cm
  2. Lvs sub-orbicular; petals pink; branches herringbone:   C. horizontalis
  2. Lvs obovate-elliptic; petals white:   C. integrifolius
1. Lvs more than 1.5 cm, at least twice as long as wide
  3. Lvs bullate, to 15 cm long
   4. Fruits black
    5. Lvs strongly bullate:   C. moupinensis
    5. Lvs not strongly bullate:   C. villosulus
   4. Fruits red
    6. Lvs to 7 cm, petioles 3-6 mm; calyx hairy:   C. bullatus
    6. Lvs to 15 cm, petioles 1-3 mm; calyx hairy only on the margins:   C. rehderi
  3. Lvs flat
   7. Lvs not tomentose below
    8. Lvs less than 3 cm:   C. simonsii
    8. Lvs more than 3 cm
      9. Petals erect; fruits red, becoming black:   C. laetevirens
      9. Petals spreading; fruits remaining red:   C. x watereri
   7. Lvs tomentose below
    10. Corymbs 3-7-fld; calyx lobes long mucronate; lvs 1.5-2.5 cm:   C. dielsianus
    10. Corymbs 5-15-fld; calyx lobes acute to acuminate; lvs more than 2 cm
      11. Lvs 2-3 cm; corymb shoots to 4 cm; fruits orange-red, oblong-ovoid:   C. franchetii
      11. Lvs 2.5-6 cm; corymb shoots to 1.5 cm; fruits subglobose:   C. sternianus