Many people do precisely what you are doing, but don't follow all of the implications. Here are a few:
1. You need to decide if the income is US or foreign source on your US tax return both for Form 1116 and for Form 2555.
2. You need to know if you should already have registered in the UK as self-employed (you only had 3 months, so will probably be fined the automatic £100 penalty).
3. You need to have registered for paying UK National Insurance.
4. If you are claiming exemption from Self-Employment Taxes you'll also want to have requested a Certificate under the totalization agreement.
5. You may need to have registered for VAT.
6. You need to prepare accounts for your US tax return following Generally Accepted Accounting Principles (GAAP), but following US GAAP.
7. You need to prepare accounts for your UK tax return following Generally Accepted Accounting Principles (GAAP), but following UK GAAP.
8. You'll need to adjust both sets of accounts for tax purposes, including such items as depreciation, capital allowances and transfer pricing.
9. From a corporate perspective the company paying you will need advice on whether your presence in the UK forms a permanent establishment of the US entity thus requiring the entity to prepare filings for UK companies house and tax authorities.
10. You will also want employment law advice as to whether your activities have the badges of trade (eg you have the risk of losing money and can engage others to complete the work) or instead you are actually an employee under English law who happens to work at some distance. If you are an employee (whether or not the payor labels you a contractor) then you have UK employment rights and will need to pay tax as an employee.
11. There is UK tax to pay for the year ending 5 April 2007. You'll need to decide when best to get this paid over.
Precisely as Lizzit says, these are all complicated issues, and it easy to be thankful for getting the work without spending the time getting all your ducks in a row.