Deathwishes are not currently legally enforcable (it's something we are looking into though).
If you do die, a list of your deathwishes will be provided alongside your payout to the beneficiary. It's kind of like an 'expression of wishes' which is often attached to a will. These are not enforceable, but you'd kind of hope that your next of kin will respect your wishes.
If your wishes have monetary value, the beneficiary will decide whether or not to spend the money the way you intended. E.g. if your bereaved wife decides not to give £100k to your mates for a massive blow out in Vegas, that's up to her.
If your wishes do not have monetary value (e.g. your mate Billy dressing as a fairy at your funeral) then it will totally be up to them whether they acquiesce or not. We would recommend sharing your deathwishes far and wide in the hope that Billy feels peer pressured into it.