There are provisions for filing previous years' taxes. You can get the forms you need at this IRS web page for "prior years forms, instructions and publications" (back to 1990):
http://www.irs.gov/formspubs/article/0,,id=98339,00.htmlThe most important thing will be to have all your records in order--particularly pay stubs and bank statements from the U.K.
The main form needed for each year is the 1040, which summarizes your income and the amounts from any other forms. After that is the 2555 for declaring and excluding "Foreign Earned Income"--because of this, you won't owe U.S. tax on what you earned unless it was a huge amount. Did you have self-employment income of any sort? If not, that may be about it for you!
I'm sorry, I don't know if there are arbitrary penalties for late filing, but I hope that since you should have no amount due, there would be nothing to penalize... ?
For exchange rates, there are UK-Y regulars who have a better handle than me on what is expected. It seems that the government may accept an annual average conversion in normal cases, which would simplify things for you a lot.
Good luck! I know it is the most appalling hassle.