There is some relevant precedent in Dutch courts over this. If you have a fine clause for spamming, and you provide the terms of conditions in a proper manner, both can be enforced by a judge.
In one case ("zaaknummer 676425") a community website got awarded 5000Euro in penalties (500Euro for every private spam message, up to maximum of 5000Euro). The website had put a penalty clause for spamming in their terms of conditions, and continued use of the service meant agreeing to their conditions.
With law it is about intent too. If you try tricks like a proof-reading service, a judge may frown on that or find it silly. But if you send back your terms of service stating: "I charge a freelance administrative fee of 50Euro for reading, evaluating, and storing commercial communication that is send to this e-mail address without explicit opt-in permission. Unsolicited e-mail can be send to this alternative address." and yet they continue, it could stand up in Dutch court (and may earn them a bigger fine for breaking Telecom laws).
Just because I define a series of steps that constitutes them agreeing with the contract I design, it doesn't mean that following those steps means they agree.
Absurd example: if the New York Times continues to publish, they agree to pay me $1000 a day. (This example is missing consideration[1] but you can imagine that in yourself. It's an absurd example anyway.)
Sure, I agree with you, the courts would most certainly not rule in favor of a judgement of £200 per e-mail, but I'm sure that's not the point.
They "consented to be billed for proofreading services" every bit as much as this person "consented to receive their spam marketing e-mails". In other words, not at any time or in any reasonable form that could have been construed as consenting or agreement.
I have no doubt the point of the joke was lost on the spammers/marketers, even if they even ever actually saw those terms and associated invoices.
If I want to make a comment on your website, and you have a form auto-checked that lets you send me spam, I have at least a choice, and it was something that, in your absence, I wouldn't automatically have a right to do.
Now the question arises: do I have a right to send things to your inbox? You certainly have created an interface with an API that I am obeying, but I've never liked treating APIs as legal constructs (although many of my fellow nerds wish it were so) so it doesn't necessarily follow.
There is one issue with lack of notification. I could say that anyone who goes on my property owes me $500,000 for the pleasure of being on my property. I could even send registered mail to every person on the country to inform them of this rule. I don't think walking on my property indicates agreement, because even if they read and internalized my mail they have no way of remembering.
It's really just posturing; hypothetically, going with your example you don't send those letters because you want or expect to get $500,000 from each uninvited guest, you send them because when you find the intruders on your property unauthorized you're going to be able to fire a round into the air and tell them "this is your second warning".
The goal is to get them to leave, or to never even come. Unless you were really trolling for $500,000. To part with that example, the law in question here specifically regarding inboxes requires opt-in, so only providing an interface to opt-out is directly against the law.
There is definitely no compatible land-owner analogy: if you greeted people at the border of your property and made them sign a declaration that they would pay you $500,000 for this temporary use of your property, you might have a case. They opted in.
I have a harder time imagining a judge who would see it your way when "you spent the $200 million required to reach every person and you said very clearly, 'your presence on my property constitutes acceptance of these terms,' so by law you should get the $500,000."
The two examples are not equivalent. One is about whether or not a contract is formed, which requires specific steps under English law (roughly: offer, acceptance and consideration).
The other (consent to receive spam emails) has nothing to do with contract law.
We can probably at least agree that two wrongs don't make a right... they are not equivalent but both are wrong according to the law, if not totally illegal. In fact I'm no English law expert, but I'd wager that while the spamming is actually illegal according to the law and the judge, the invoices GP proposed are simply unenforceable and not actually against the law.
In one case ("zaaknummer 676425") a community website got awarded 5000Euro in penalties (500Euro for every private spam message, up to maximum of 5000Euro). The website had put a penalty clause for spamming in their terms of conditions, and continued use of the service meant agreeing to their conditions.
With law it is about intent too. If you try tricks like a proof-reading service, a judge may frown on that or find it silly. But if you send back your terms of service stating: "I charge a freelance administrative fee of 50Euro for reading, evaluating, and storing commercial communication that is send to this e-mail address without explicit opt-in permission. Unsolicited e-mail can be send to this alternative address." and yet they continue, it could stand up in Dutch court (and may earn them a bigger fine for breaking Telecom laws).