Lexus dealers offer a fixed-price deal for older cars such as the RX400h. They charge £395 for rear shocks.
http://www.lexus.co.uk/owners/servicing-and-maintenance/#ServicingYourLexus
Other than that, Rock Auto has Monroe struts at £74.55, and KYB struts for a bit less. They ship from the US, so you will have to add shipping and import costs.
http://www.rockauto.com/en/catalog/lexus,2006,rx400h,3.3l+v6+electric/gas,1431837,suspension,strut,7584?a=Referer+www.google.co.uk+URL+%2F%3Ffrom_co_uk