Whether or not you are readmitted will be totally at the discretion of the immigration official. In addition to your onward ticket and proof of ability to support yourself without working, be sure you can demonstrate "a compelling reason" to return to the UK or to go to some other country. This is above & beyond the onward ticket.
Nowadays, I-94 forms are tracked and "losing" it will indeed get you on an overstay list.

#6 If you don't turn in the I-94W (note NOT the I-94 which is a totally different form) the assumption will be that you haven't left the US.
It is then UP TO YOU to prove that you left the US before your visa Waiver expired before you can return to the US

The only thing I can think of is that because she is a Permanent Resident in Canada, they don't apply the rule vigorously.
It's not that they don't apply it - it's that it's a slightly different rule for bona fide permanent residents of Canada. Returning to Canada for them counts as leaving the US.

Let's save the OP some time and start discussing how she can get from Costa Rica to the UK without transiting through the US.

#21, the rules are different for Canadians, Mexicans, Bermudans, and permanent residents thereof.

I'm afraid it's a little late to try to dim the OP's over-confidence in her approach, but she should realize that it's not really relevant whether her passport gets stamped if she's refused entry ("Why, I'll just get a new passport, then!"), because the form filled out on entry to the US asks whether you've ever been denied entry to the US, and it'll be in the computer anyway.
If she's denied entry she might be returned direct to her home country, rather than Costa Rica. But if she is sent back to Costa Rica, then she'd probably have to apply for a Transit Visa at the US embassy there, given the few alternatives for returning to the UK without a transit in the US.
Let's hope she doesn't forget to pack her ticket back to the UK: she will need it to get on that plane from Costa Rica.
Her alternative -- which no one mentioned -- would have been to apply for a regular tourist visa in Costa Rica (or the UK, etc.) and not try to enter on a visa waiver.

#26- She does not get returned to her country of origin. She is returned to the country from which she boarded the plane, by the airline that she arrived with. If they have a flight with space, the airline flies her back then. If not, she waits in a detention room for days until the airline has an available flight.
She cannot apply for a B1/2 tourist visa in CR. She must apply in the country whose passport she uses. Likewise for a transit visa
If she attempts to break the rules and is detected, she will have to figure a way that does not fly her through a USA airport.
I'm with VinnyD #24. My best advice is to have her flights back out of CR to UK figured out.
She cannot apply for a B1/2 tourist visa in CR. She must apply in the country whose passport she uses.
Not exactly. <blockquote>Quote
<hr>Where Do I Apply for a Visitor Visa?
Applicants for visitor visas should generally apply at the American Embassy or Consulate with jurisdiction over their place of permanent residence. Although visa applicants may apply at any U.S. consular office abroad, it may be more difficult to qualify for the visa outside the country of permanent residence. <hr></blockquote>
According to the State Dept. there is currently a 25 day wait time to get an appointment for a visitors visa in San Jose. You can call the embassy & plead emergency to get a quicker appointment, but it has to be pretty dire.
Iberia flies nonstop from San Jose to Madrid. Air Canada flies to Toronto. She can also get a flight to Mexico City and fly from there to other European cities & get to London from there. None of these flights come cheap.