The best way to buy a car is to take the deposit contribution for finance and then pay the lot off after 14 days. That way you get the contribution (although its still best to read the small print) and only have to pay a small proportion of interest.
The interest isn't exactly low rate. Even if you pay 3% on a £53k GS450h, it'd cost you an extra £1.5k a year or thereabouts as you pay it off. Eventually it works out £5-7k dearer.
If you change your car on a regular basis, the 40% you talk about is depreciation, thats it. You dont own the car. Its worth noting that every stonechip, every wheel scuff and every scratch is taken into account...... at DOUBLE the price it should cost to fix when you hand the car back. Therefore, you end up actually putting about 60% in for 3 years or so.
You talk about putting 100% in. Thats not entirely true. You put 100% in when buying the car, when you sell it you get 50-60% back after 3 years so it costs 40-50% instead of 60%. When you take the £54k into account the 10-20% difference is huge!