If you referring to the LS430 than both 04 and 05 are same tax - there are few 06 plated LS430s which fall into the higher tax bracket along with all LS460s. The LS600h are lower tax due to being a hybrid (although they have the biggest engine in the line up)
back to topic on ls430 prices - they have recently seen a rise especially lower mileage ones
this one was snapped up from a private seller - i think it was priced around 5k:
2004 Lexus LS 430 4.3 4dr | eBay
Kass classic have an ls430 for £6990 2004 Lexus LS430 4.3 4dr Full Lexus Dealer History For Sale | Car and Classic
or you can choose to have the ls400 for £7990 from the same dealer 1999 Lexus LS LS400 1 Former Owner 90,000 Miles Facelift For Sale | Car and Classic
A very low mileage LS460 also commands a premium price £24K??? - compare the prices between these 2 - would you pay extra for the lower mileage:
2006 Silver Lexus LS 460 4.6 SE-L 4dr for sale for £9495 in Edinburgh, Midlothian (autotrader.co.uk)
14k miles LS460 2009 Blue Lexus LS 460 4.6 SE-L 4dr for sale for £24990 in Cobham, Surrey (autotrader.co.uk)