hi linkmani am not an accountant
but
my understanding is that you can only claim a tax loss if the shares are sold
however if the company goes into receivership then you have to wait till the receiver completes winding the company up and pays out money or a portion of the money owed then if there is any left over(which is highly unlikely)
will be distributed to share holders that could take years and only when that is done can you claim a loss or a capital loss
there are a few accountants on this site that may be able to give more info.
this is not tax or financial advice
it is only info i have picked up from the pub
dyor
cheers nb1