For a connecting flight, will United need my final destination visa?

Visa on arrival for Canadians is only available if:

1. You are invited by a company registered in Pakistan (and this requires prior approval); and your stay has to be for business purposes or;

2. You are part of a tour group and traveling through a designated tour operator.

Of course this isn't important as you are a Pakistani citizen and cannot be refused entry.

As far as United is concerned it depends on how you have booked the ticket. If it is one ticket they are well within their rights to refuse you boarding.

If your United ticket is just from NY to London, then they cannot refuse you as your Canadian passport allows you entry into the UK. They won't care about your next leg.

The NICOP (National Identity Card for Overseas Pakistanis) allows you visa free entry but you need to convince United of this and of the fact that you will have it in your possession when you arrive.

If you have a copy of the (new NICOP) you can ask for an emergency travel document at Pakistan embassy - although they might wonder why you don't have a Pakistani passport.

According to United's Contract of Carriage:

Rule 19 Travel Documents
Each Passenger desiring transportation across any international boundary is responsible for obtaining and presenting all necessary travel documents, which shall be in good condition, and for complying with the laws of each country flown from, through or into which he/she desires transportation. Any Passenger who, by failing to comply with the laws of each country flown from, through or into which he/she desires transportation, causes UA any loss, damage or expense of any kind, consents and acknowledges that he or she shall reimburse UA for any such loss, damage or expense. UA is not liable for any assistance or information provided by any employee or agent of UA to any Passenger relating to such documents or compliance with such laws, or for the consequences to any Passenger resulting from his/her failure to obtain and present such documents, which shall be in good condition, or to comply with such laws. Where legally permitted, UA reserves the right to hold, photocopy or otherwise reproduce a travel document presented by any Passenger. UA also reserves the right to deny boarding to any Passenger whose necessary travel documents are not in good condition according to UA's reasonable belief, or which otherwise do not comply with laws of the specific country the Passenger is departing from, transiting through, or traveling to.

I couldn't find a similar visa related thread over on flyertalk, but there are similarities with this passport issue:

http://www.flyertalk.com/forum/delta-air-lines-skymiles/1592140-idb-dl-denied-us-boarding-domestic-flight-due-no-passports.html

There are a number of speculations about the reasons for denying boarding on a first leg when documents could later be procured for a following leg, such as the responsibility of returning the passenger to origin if they cannot board the later flights. However, I suspect this is just policy and that a document check flag will be set for this itinerary and must be completed prior to boarding the first flight.

It's possible that you might not have to, but in the absence of anyone saying they've managed to do the same thing, I'd prepare for the worst and consider back up plans.

Logically the outcome of your situation ought to be that United in New York will be able to issue you a boarding pass for the first leg, but not for the second -- and then you'd be free to check in for the second leg in London once you have arrived there and acquired the appropriate paperwork. You probably can't get your baggage checked through either, but it sounds like you'd have time enough to lug it around during your London layover anyway.

After all, United won't be in trouble with anyone for flying you to London; they can verify that you're entitled to enter request leave to enter there, and if you get stranded in the UK because your plans to acquire Pakistani ID there fall through and you're denied check-in for the second leg, that will not be United's problem, only your own.

However if you have called the airline themselves several times, and even they cannot get their own story straight, then you don't really benefit from being told by random people on the internet what we think the logical outcome should be. Even if we're right, based on reading the rules, that won't help you much if the check-in agent you encounter belongs to the faction within United who thinks they need to see travel documents that are valid all the way to your final destination.

(Personally, based on your information, though, I think I would chance it -- and hope the check-in agent can see the reason in your plans. How sure are you that those you talked to who told you otherwise understood fully that you're okay with only being checked in for the first leg from the beginning?)

I'm just posting the result of my experience in case anyone else somehow ends up in the same situation and has no options but to cancel the ticket or try traveling.

United Airlines didn't bother checking for a visa at all. In the UK, PIA made sure to check my Pakistani ID several times. I did thankfully have my ID with me before leaving the US (thanks Fedex), but I never showed it to UA. I also only got one boarding pass in the US.

My guess is that United has really low transfer volume to Pakistan and can safely leave the check to the last airline (maybe they have an agreement to?). Maybe also, because entry to Pakistan is allowed (and common) without a visa but an ID card, and they aren't equipped to check those.

However, I think that my situation was exceptional. The other answers posted are far clearer and better researched than what (several) United Airlines' own support staff could come up with.

Final destination visas should be ready before any travel - my type of situation should be avoided as much as possible (I ended up in it at last-minute because of a bad date mix-up).

As a sidenote, I also recommend trying to get a hold of UA staff working in the airport, or visiting the airport and talking to them directly, maybe on a similar flight a few days before. In retrospect, I should've asked the UA T1 call-centre support in the Philippines to transfer me.