Your initial rental is calculated within the depreciation of the vehicle, factored into the arrangement, so it isn't a deposit and doesn't get returned.
As an example let’s look at a 24 month lease on a £100 monthly rental – If you chose an initial rental of 6 months, your first upfront payment in the first month would be 6 x £100 = £600. You’d then pay the remainder, £100 monthly, for 23 months. (£2,900 overall.)
If you choose to pay a bigger initial payment, for instance 12 months upfront, you’d pay more upfront in your first month, but less per month for the remaining 23 months. Using the example above, you’d still be paying £2,900 overall though.
Think of your lease like sharing a cake between 24 people – if the first person wanted a large slice, the other 23 would have smaller slices, if the first person wanted a smaller slice, the others would have more each. Overall though, whatever initial rental option you take, the total rental cost would be roughly the same, so how much you pay initially is up to you.