You say you don't want to lose anything. If you mean that you want to have 100% guarantee that - in 15 years time - you can take out at least £100 for each £100 you put in now, then the only way I know of doing this is to use building society or bank savings accounts. BUT 15 years is a long time & the value of each £100 you take out at the end of it will be much less than its value now, because of inflation. And the interest is most unlikely to bring in enough to cover the effects of inflation.
This is why it is generally recommended that equity investments of some kind are used for any long term savings - on the basis that they outperform cash savings over a long period.
The trustees really must take professional advice on this, otherwise they could be open to claims by the beneficiaries that they had failed in their fiduciary duty to them.