Jose,
Alas, doing things and then getting your advice is a bit pointless. Taxes are determined by facts and circumstances. Once you've done something, the number of options you have are severely narrowed. The time to seek your advice is before you've done something, so you can maximize your options.
You'll pay at your regular rates and get whacked with tax in the USA. If you had waited until next year, your US tax would be under $300. How much you pay in the UK depends on your residence and domicile status for 2005.
US years are Jan-Dec. If you are a greencard holder, that's what you'll be doing. If you are not a greencard holder, you can file as Dual Status Alien (DSA) for just the period you spent in the USA. DSA returns and greencard abroad returns are very complex. You should probably seek an international tax preparer who specialises in this sort of tax return.
UK years are Apr-Apr. You may use a short year if you arrived mid-April, but it will be effectively the same.
The answer to your original post was quick and easy. The answers to your second post would take me about a half an hour, and be more substantial in nature. Space and time does not permit me to answer fully.
If you're looking for good starter sources of free information for the US, try
http://www.irs.gov/businesses/small/international/index.htmlFor the UK try
http://www.taxationweb.co.uk/guides/residence_and_domicile.php - another place where I post replies to tax queries.