Tyres.co.uk has the Bridgestone Turanza 6 and Michelin Pilot Sport 5 in 215/45 R17 for the UK IS 300 Saloon (2001-05). The SportCross has different rear fitments, so check the placard for the size, load index and speed rating before buying.
To find the tyre size placard for the IS 300, open the driver's side door and look at the B-pillar. There is a placard that has the front and rear fitment information for the IS 300, as well as the specifications and pressures for each. For SportCross models, do not rely on the information on the tyre, go check the placard.
Lexus IS 300s do not need all four tyres to be replaced, the UK IS 300 models are RWD with no AWD version requiring all four wheels to be replaced. Tyres should be replaced as a pair for each axle and if new tyres are being fitted, they should be fitted to the rear axle.