I am also interested in art law. I have used the Legal 500 and other law guides as a basis to decide what law firms I am applying to.
You probably have already had a look at these firms, but just in case, the following firms are Tier 1 in the Legal 500 for art and cultural property law:
Boodle Hatfield
Farrer & Co
Charles Russell Speechlys
Mishcon de Reya
Stephenson Harwood
Hope this helps!