Cashing in endowments



I need some advice on cashing in an endowment

I have been offered an amount from the insuarance company but this amount means any anniversary bonuses to date will be lost

The policy has been in existence since 96 and are due to run until 16

The bonus is worth about half of the surrender value at present

Is there anyone who buys these with a view to holding onto until maturity, and if so how would they calculate the value comared to surrentder value?