Liberals are not leftists ffs. S&D are liberals, if you go to the wiki page you can see that they are labeled centre-left. Centre-left in this case stands for left wing of liberalism.

https://en.wikipedia.org/wiki/Green_Party_of_England_and_Wal...

This is an example of party which can be called leftist. It's actually even listed as that on wiki. Minimal condition in general is to be anti-capitalistic.