I'm looking for the full text of (or a link to) a scathing letter
written by Percy Bysshe Shelley to the father of a lady-friend he had.
Shelley's relationship with the woman was non-amorous, but her father,
the proprietor of an inn and a brute of sorts, refused to believe it
and prevented him from seeing his daughter.

The eloquently written letter ended something like this:
"Neither the laws of nature nor of England have made children Private
Property".

My guess is that this is one of the letters Shelley wrote
to William Godwin.  Godwin tried to prevent his daughter
Mary from seeing Shelley.  Shelley protested to no avail.
Eventually Mary and Shelley eloped.  The letters between
Shelley and Godwin are easy to find in print, but I didn't
have much luck with a quick internet search.  Hope this
helps in some small way.

Thanks for your comment, I appreciate it... but I doubt the woman in
the letter was Mary Wollstonecraft Godwin Shelley.

Mary Shelley's father was William Godwin, a scholar of sorts
http://www.spartacus.schoolnet.co.uk/PRgodwin.htm

The father of the woman in the letter I'm referring was some sort of
retired bandit. And Shelley did not elope with her -- their
relationship was non-amorous.  And I remember that this woman was not
the author of Frankenstein... she was a school teacher, I think...
(don't remember)

Possibly it's in "Shelley's Prose" ed. D.L.Clark 1966

Here it is at last:
PERCY BYSSHE SHELLEY to Thomas Hitchener, 14 May 1812
Percy Bysshe Shelley, 1792-1822; Thomas Hitchener, father of Elizabeth
Hitchener. Shelley had formed an intense friendship -- which was
intellectual rather than amatory -- with Elizabeth Hitchener, who ran
a school near Hurstpierpoint. Her father was a retired smuggler who
kept a public house.

                              Nantgwillt, May 14. 1812
Sir

If you have always considered *character* a possession of the first
consequence you & I essentially differ. If you think that an admission
of your inferiority to the world leaves any corner by which yourself &
character may aspire beyond it's reach, we differ there again. In
short, to be candid, I am deceived in my conception of your
character.-

I had some difficulty in stifling an indignant surprise on reading the
sentences of your letter in which you refuse my invitation to your
daughter. How are you entitled to do this? who made you her governor?
did you receive this refusal from her to communicate to me? No you
have.- How are you then constituted to answer a question which can
only be addressed to her? believe me such an assumption is as impotent
as it is immoral, you may cause your daughter much anxiety many
troubles, you may stretch her on a bed of sickness, you may destroy
her body, but you are defied to shake her mind.- She is now very ill.
You have agitated her mind until her frame is seriously deranged -
take care Sir, you may destroy her by disease, but her mind is free,
that you cannot hurt.- Your ideas of Propriety (or to express myself
clearer, of morals) are all founded on the consideration of profit. I
do not mean money but profit in its extended sense:- As to your
daughter's welfare on that she is competent to judge or at least she
alone has a right to decide. With respect to your own comfort you of
course do right to consult it, that she has done so you ought to be
more grateful than you appear.- But how can you demand as a right what
has been generously conceded as a favor; you do right to consult your
own comfort, but the whole world besides may surely be excused.

Neither the Laws of Nature, nor of England have made children private
property.-

Adieu, when next I hear from you, I hope that time will have
liberalized your sentiments.

Your's truly
  P.B. Shelley