My response is always something like "Thank you for acknowledging my contribution to the innovation to the recruitment industry, i'm very proud to be a pioneer in this field, being first is a great honour. One thing to note, i'm not opting in, i'm refusing to opt out, i dont need to opt-in to the law. I've chosen to do this because theres no reason to and the law specifically protects me from you forcing me to do so. Also, since your client has already decided to hire me, even if i was to opt out, it wouldnt be recognised in court as the opt out paperwork must be signed before i ever meet the client."
For this point on the agency regulations, i do it primarily just to make the recruiters work for their exorbitant markup on my day rate. However i do also question source code and copyright clauses like the author did, almost always, i'm looked at with dumbfoundment that i even bothered to read the contract.
If anyone wants to know the regulations i'm referring to, heres a link: http://www.contractoruk.com/agencies/5158.html