Hi,
Ok, if you're married to a USC (as opposed to a permanent resident), and if you entered legally originally, then you're ok despite the long "overstay". (Overstay will be forgiven on marriage to a USC, with the proper filings, but illegal entry won't be.)
Personally, I would go to the nearest UK consulate in the US and get your passport renewed. You need a passport number for the forms, and it might as well be the current passport, I would think.
Keep the old one though! It's vital to have evidence that your initial entry was legal! Do you still have your entry stamp from your original entry, and/or an "I-94" card stapled into the passport?
As far as I know, you'll never see the "visa number". It'll be used in the dark mysterious depths of the CIS while processing your AOS (adjustment of status) application. What you'll eventually get is a stamp in your UK passport which will be your first temporary "green card". Then a permanent plastic green card (which won't be green) will eventually come in the mail
FWIW, I like this lawyer, and personally I would get her do-it-yourself AOS kit if I were starting over, rather than asking a million questions on forums about how to file: "Marriage to a Citizen When the Applicant Is Present in the United States":
http://www.visacentral.net/diy.html. It includes unlimited email questions to her for 30 days after purchase. I had a phone consultation recently with her about a more complicated issue, and was pleased. See the list of "exception conditions" at the bottom of the page. If any apply for you, then get a consultation with her.
For background info, see "Sponsoring a Fiance or Spouse for a Green Card"
http://tinyurl.com/34m9q (same as
http://www.nolo.com/lawcenter/ency/article.cfm/objectID/D9AB2E3C-DD5E-4203-AE4702E527005C37/catID/C08A0295-9AFE-4F69-A9B7AEE732ECA9AB) .
You're welcome! I got lots of help online when starting out with this and am happy to pass it along when I can.
Joe