Yes our House of Lords and Commons and your Congress sound very similar though there are key differences. Only the House of Commons are elected. They are elected every 4-5 years by the electrate through a General Election. None of the Lords are elected by the people, they are either appointed by the ruling Government or are Hereditary Peers, as in a son will adopt their father's Peerage. That means that the Government could give a Peerage to whom ever they wanted- usually a person that supports their party, as Blair is doing now. We, the electrate, can do nothing to stop this. A new law- called a Bill- has first to be proposed in the House of Commons. The elected MPs theb have to vote on it, whether they agree or disagree with it. If it passes the Bill is sent onto the House of Lords. They then scruntinise it. They can send it back to the House of Commons with amendments but only twice. They cannont refuse it- if they try to (as happened with our recent Fox Hunting Ban) Government can use an Act of Parliament. This means that the House of Commons, the superior House, enforces the Bill regardless of what the House of Lords thinks.
Yes Britian has a Constitution but it is uncodified- ie not written down on one peice of paper. Our constitution exists in various sources: Statue Law, Common Law, convention, texts and the EU.
The Government has no control over the Judical Branch of Government, it remains complelety neutral and independent. The Government cannot appoint Juges to civilian cases but can call on them to chair important commissions or enquiries, such as the Butler Report ( the offical report into the British government's handling of the Iraq war.)

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"Never has so much been owned, by so many, to so few."