Short Date into English Long Date

Python 3.6, 137 129 bytes

from datetime import*
def f(k):g=[*map(int,k.split('-'))];n=g[2];return f"{date(*g):%-d{'tsnrhtdd'[n%5*(n^15>4>n%10)::4]} %B %Y}"

Try it online!

Python 3.6, 154 bytes

from datetime import*
s=[*map(int,input().split('-'))]
b=s[2]
print(date(*s).strftime(f"%-d{'th'if(3<b<21)+(23<b<31)else('st','nd','rd')[b%10-1]} %B %Y"))

Try it online! (Set input stream and then run.)

Thanks to good suggestions from commenters below.

JavaScript (ES6), 142 140 bytes

Outputs NaNth Invalid Date for invalid dates.

The code for ordinal numbers was adapted from this answer.

d=>`${s=(D=new Date(d)).getDate()+''}${[,'st','nd','rd'][s.match`1?.$`]||'th'} `+D.toLocaleDateString('en-GB',{month:'long',year:'numeric'})

f=
d=>`${s=(D=new Date(d)).getDate()+''}${[,'st','nd','rd'][s.match`1?.$`]||'th'} `+D.toLocaleDateString('en-GB',{month:'long',year:'numeric'})

console.log(
  f('2005-12-3'),
  f('1980-05-12'),
  f('2005-12-3'),
  f('150-4-21'),
  f('2011-2-29'),
  f('1999-10-35')
)