realistically you'd have to calculate how much money you are paying sooner rather than later and calculate it on an accrual basis over the period. interest rates might be low right now so if you had no debts then based on those rates the interest may be low, but if the interest rates increase it would be higher
but if you were buying the phone outright on credit, or you had credit elsewhere that you could have reduced by organising your cashflow differently, the interest is typically higher on credit right now, so could be on average 8% to 10% for the period, thus paying £400 more would cost you £32 to £40 in interest
in real terms if you are not so financially well off and you have debts, it would likely cost you more to pay upfront and then simplicity or PAYG
i think most people are aware that PAYG of any kind means you pay more and get less value, whether it's mobile phones or fuel bills, but the same can apply to sim free purchase
this is pretty simple maths for the numerically minded