Finding Shares

My mum recently passed away and I am trying to deal with her estate. Can anyone advise on how I can find out what shares she may have owned?

I know she had BT and British Gas shares but think she may have cashed them in when my dad died. In addition, there are payouts on her most recent bank statement marked ‘Lloyds’. The lady at the Halifax said if she had had shares with Lloyds they would have shown up on their system when I stopped her accounts. I can’t find any paperwork other than the bank statements.

Thank you.