Yeah it can all be a bit head bending.

How is your car currently financed?
I can't see how you will fail to pay at least £1k deposit if you are changing your car every 2 years!
It may well be beneficial for you if you work out the full cost of ownership over 2 years for each method as this will tell you exactly how dear / cheap the deals are. You might very well be paying a low deposit but higher monthlys or more over the term.
My last 2 PCP's gave me about £1k equity to put down for my next car.
One other thing about leasing which maybe someone that leases can clarify, I read if you aren't the registered keeper which I'm not sure you are on a lease then a lot of insurance companies won't insure you so possibly paying more that way.