£3750 trade in? For what?

[oldmanbm]

I have an old Renault Megane that I used for tip runs and leaving overnight at the station, It's still taxed and MOTd and I've had it for five years. Now approaching retirement I was looking at a new Tesla Model 3 with good trade in and 0% finance. My brother-in-law will give me good price for my four year old Model 3. I tried to buy a new Model 3 using the Megane as a trade-in but it would not generate more than £365. Do I need to further in the deal for the £3750 to be applied ie. after finance agreed?