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I think you have seriously screwed up your I-485 application and you should really have thought about the implications when you accepted a job in the UK.
Generally, an I-485 application is considered automatically abandoned and invalidated if you leave the U.S. while I-485 is pending without getting an advanced parole first. As I understand you do nor have advanced parole; the only way to apply for it is from inside the U.S.
The only exception to this rule is for people who travel abroad during their I-485 process while maintaining a valid H/L status in the U.S. Maintaining status means having a valid H or L approved petition and working for the H or L sponsoring employer. In this case you can apply for an H/L visa stamp at a U.S. consulate abroad and re-enter the U.S. using this visa without advanced parole.
As I understand it, you quit your american H1 employer (or were fired) and are not employed by this employer now. So you do not have a valid H1 status now, no matter whether or not the employer bothered to cancel your H1 petition.
You cannot apply for a visitor or a student visa etc since those visas assume non-immigrant intent. Having applied for a GC, you now established an immigrant intent and thus are ineligible for anything other than H or L visa (they have dual intent). If you try to apply for a visitor visa, you\'ll be denied. If you lie on the visa application (always a bad idea) and somehow get a visitor visa and use it to enter the U.S., you\'ll have a problem with the stamping. At the moment of stamping they\'ll want to get your last I-94. If you show then I-94 with B-1 status indicated on it, you are certainly not going to get a stamp (could get detained and deported in fact).
And most certainly you cannot get your passport stamped with GC at an american consulate by showing them an I-485 approval notice. Contrary to what Radek says, consular processing is not something that can be used in conjunction with I-485 process. It must be either one or the other.
So the way I see it, there is no legal and honest way for you to get in the U.S. now. In case your original H1 petition has not been cancelled or expired, you can try to get an H1 visa stamp abroad and re-enter U.S. using it. However, you\'d have to lie on the visa application since you are not working for your H1 employer now. You might not get caught, but it is a serious risk.
The only legal alternative I can see is for you to notify INS that your I-485 application has been abandoned and that you request your I-140 approval notice to be transferred to the National Visa Center for consular processing. Then you\'d have to start consular processing from the beginning.
The above is my opinion. Given the fact that your I-485 application has been screwed up so seriously, you certainly have to consult a good immigration lawer.