This something may be innately obvious to native speakers; however, to many outsiders, the difference is elusive. I only recently realized the difference and still have a hard time to distinguishing them. Please explain how to pronounce these two letters correctly, specifically lip/tongue movement (no pun intended).
W: With your tongue at rest (not touching teeth or lips, etc), pucker your mouth into a small "o" as if you are going to whistle (only not quite so hard, or it will tire you out), and let your voice take over.
V: With your tongue at rest, pronounce a voiced "ffffff" sound. My enunciation of this proceeds by putting my top teeth against he inside of my bottom lip and blowing. I voice it for a "v" sound, meaning that my vocal cords vibrate making for a noisier, louder sound..
I realize that different people pronounce it differently. For the record, I am a native American-English speaker living in Pennsylvania in the United States. (Not a native American, to be sure.)
Take a look at these interactive phonetic animations. Sadly, I am not aware of any similar that feature British English, but if you choose American English, you will gain access to an excellent resource of phonetic animations with sound and frontal videos of people during pronunciation, sorted according to the International Phonetic Alphabet (IPA). V is the voiced labiodental fricative (consonants –> place –> labiodental –> /v/) and W is the voiced labiovelar approximant (consonants –> place –> bilabial –> /w/). You could also choose to see the process explained chronologically. I do not think that there is a better explanation, really.
Turning my comment into an answer:
For British English, this video produced by the BBC, explains the difference: http://www.bbc.co.uk/worldservice/learningenglish/grammar/pron/sounds/con_other_7.shtml