>>>If someone is left some money in a will but dies before they get the chance to recieve it
It's not entirely clear whether you're referring to a beneficiary of a will dying before the testator does, or whether you're referring to the testator dying first and the beneficiary dying shortly afterwards (before they've received their share of the estate).
Second one first ('cos it's easiest):
If the beneficiary dies while waiting to receive their inheritance then that inheritance still forms part of the beneficiary's estate (and is then distributed in accordance with the terms of their own will, or the intestacy laws, as appropriate).
Now the trickier one:
A well-drafted will should state what should happen if a named beneficiary dies before the testator. (The money could be given to someone else entirely, given to a relative of the beneficiary, 'added to the pot' for the surviving beneficiaries, or anything else that the testator desires).
If the will does not specify what should happen if a beneficiary pre-deceases the testator, then what will happen depends upon whether or not the intended beneficiary was a son or daughter of the testator. If so, the gift under the will is shared equally between their children, if any. (under certain circumstances, grandchildren can also inherit here).
Otherwise, any specific legacies or bequests ('£1000' or 'my diamond brooch') are added to the pool (if any) to be shared out among other beneficiaries. Anything which isn't covered under the terms of the will is treated as creating a partial intestacy, with the normal intestacy rules applying.