Ok, dunno if I'm working this out right but according to the [url="http://www.parkers.co.uk"]Parkers[/url] web site, a 2000 IS200 Sport on a W plate with 30K miles on the clock will fetch you £14,440 private (£12,825 trade). To buy the same car from a dealer (in the colour you want!) will then set you back approx £15,190 according to the guide.
So if you sell privately, then you're looking at £750 as a best case to then trade up. More like a grand if you get knocked down, or the dealer adds a bit on.
Mind you, there's a W reg (2000) Platinum Ice 200 Sport with 27K miles going for £14,950 on the [url="http://www.lexus.co.uk/approved.php?area=hitlist&model=11&variant=Sport&transmission=1&price=&mileage=®ion=&metacolour="]Lexus[/url] website. Good luck!