No 10 Downing Street has a website where you can create and sign e-petitions. If a petition reaches over 200 signatures by the time it closes, the government have committed to publishing and emailing a response.

Current popular petitions include:

"Professional Status For Engineers and Engineering"
"Halt plans by the OCR to abolish the Ancient History A-level"
"Reject any proposals to ban or restrict the use of older cars"
"stop the Home Office from interfering in the negotiation of police pay"
"Make Jeremy Clarkson prime minister" (over 45,000 signatures! for this one)

http://petitions.pm.gov.uk/

Also check out Write to them, another website where you can get your point across to those in power. This lets you find out who your local elected representatives are and send them emails.