Try this site, very handy for finding part numbers.
I don't know the RX300, but it looks to me like this diagram shows the sensors, they're labelled as "Sensor, Air Fuel Ratio", and it looks like the same part (89467-48011) is used in two different places. As I say though, I don't know the RX300 so please don't just take my word for it ! The price is showing as around 200 Euros, so it does sound like it might be that £180 part.
A Lexus/Toyota spare parts guy should be able to tell you the part number anyway (they look on a system with these same diagrams).