it depends on your circumstances as which is the best way to buy,
the best way is cash but not many people have £20,000 + laying around. i would try getting quotes from your bank and lexus to compare apr rates, you could also consider a pcp which are becoming more and more popular, and are a good idea if you like a new car every 2/3 years.
when you collect your car they will try and sell you gap insurance which will be around £600 after a few times of saying no i got it down to £300 but decided against taking it out.
The bigger the depsot the better as this will reduce your payments, and haggle on the pice of the car they have around 5% in the cost of a new one which they can knock off, i got around £1800 of mine