@bruno: what's the average fare you pay for a roundtrip? you seem to have some kind of experience  
Mhhh... just an example, I would say ;
EuroTunnel : 70 - 80€  (for a car and without trailer... just double the price if you have a trailer)
Fuel : you can calculate ... but my average distance is ofter 1000km... Hopefully my car doesn't need a lot of fuel, and fuel is cheap in Luxembourg  

If you go further than London, you probably will pass the 
Dartford Tunnel... and so, you'll have to pay 1 or 2£ each time ... cheap
ArcadeLifeStyle things : Burger King, Dr Peppers, chips... any food... up to you  

So as you can see, it's not too expensive... but it may be a far trip for some of you, and also a little bit dangerous, because driving on the left side of the road may be a little bit 'disturbing'... you'll have to be focus !  

 Hopefully my wife keep an eye on the road all the time and she never stops to say me : be careful about this... pay attention to that ... blablabla...  
 
  
 
Roadtripin' is priceless  
 
  
 
 ...but most of you already know that  
 
  
  
 