Some are actually quite convincing,but most people don't speak with an rp accent over here. We need more cornish bad guys in us films - it will be hilarious to see how yanks can imitate that.
If you think the BBC isn't the only biased TV news channel,I have news for you - the whole of the broadcast media here can be utterly biased in various aspects of reporting events outside the country.
Its why you need to read sources from all over the world,including left and right wing to get a good feel of events. Some of the stuff the UK media conveniently ignores,is kind of eye-opening.
Edit!!
An example is the constant coverage of Syria,but the near absence of coverage of what is happening in Yemen. Also in other countries where one side in a conflict,or maybe one parties views are covered more than other views in a country,leading to a lopsided view of what is actually happening.