What does the law say about putting a license on use of things like this "Embedding this content into your site without written permission from the author will involve a £100000 fee for it's use. You agree by embedding this content that you abide by these terms." Is something like this enforceable?
With a compulsory license, the fee is pre-determined by law or arbitration. However, in the US they are basically only applicable to music and similar works involving royalties.
In cases of piracy, damages are sought through civil suit and ultimately determined by the court, not the plaintiff. Essentially your warning amounts to "I'll sue you for X amount if you steal this". There's no guarantee that the court would find in your favor or award the full damages sought.
I say "warning" because I doubt many courts would consider this a valid EULA to begin with. Contracts of adhesion are generally enforceable only if the terms conform to the reasonable expectations of the signatory (who has no control over the terms).
But even if it were valid, and you were allowed to set your own violation fee, you would still have to go through the court to enforce it.