I say good for you , in your search to find all you can about the person named in the painting (looks like a real painting to me too) , gerspee !
Sometimes it is the person who will not take 'NO!!!' for an answer that winds up finding the "YES!!!" !
Keeping in mind that it was not uncommon for folks to 'claim' to be royalty .... you might consider researching residence/census/property/tax records of the general era , in London , to see if there is any proof of such person there , way back then ...