Northwestern Europe (including northern Germany and France) has/had about 145 species.
I have seen 57 species in the Netherlands, which is not a "top list" (I guess 10 more might be); 102 in the Netherlands + northwestern Germany. The only ones I've seen in the UK (where I have seen 52), but not yet in NW Europe are Lulworth Skipper (possible in Germany) and Northern Brown Argus (which doesn't occur anywhere near). I have 13 new species left within a five hour drive (which would take me to northern Bavaria).
A whopping 17 species are extinct in the Netherlands, but most of these are still gettable in nearby Belgium or Germany. I am currently failing to find two Pyrgus species in Germany which may have succumbed to the three-year drought...
Ones that occur regularly in the Netherlands which are absent or very local in the UK:
Mallow skipper, Carcharodus alceae (new; I've only seen it in Germany but it's rapidly increasing)
Chequered skipper, Carterocephalus palaemon
Large chequered skipper, Heteropterus morpheus (strong decrease)
Cranberry blue, Agriades optilete (almost gone)
Mazarine blue, Cyaniris semiargus (local)
Large copper, Lycaena dispar (very local, endemic ssp)
Sooty copper, Lycaena tityrus
Alcon blue, Phengaris alcon (strong decrease)
Dusky large blue, Phengaris nausithous (one location, terrible disturbance this year...)
Scarce large blue, Phengaris teleius (reintroduced; never tried, but have seen it in Germany)
Ilex hairstreak, Satyrium ilicis (strong decrease)
Map, Araschnia levana
Cranberry fritillary, Boloria aquilonaris (almost gone; I've seen it in Germany)
Lesser marbled fritillary, Brenthis ino (new arrival; I've seen it in Germany)
Niobe fritillary, Fabriciana niobe
Tree grayling, Hipparchia statilinus (almost gone)
Queen of Spain fritillary, Issoria lathonia
Large tortoiseshell, Nymphalis polychloros (increasing)
Eurasian swallowtail, Papilio machaon
Southern small white Pieris mannii (new arrival, I've only seen it in Germany but it's rapidly increasing and a pain to find!)
I expect Oberthür's grizzled skipper (Pyrgus armoricanus) to arrive shortly (it's not too hard in Germany, where it is moving north at a fast pace).