You can use savings in referencing.
Where savings are separate from sources of income, you should not include savings when adding your income sources. Instead, you will have the option to include savings after you have used Open Banking to connect the bank account(s) that your savings are in.
Unfortunately, we cannot take investments into account when making our referencing recommendation, nor can we accept bank statements or other documents as proof of savings.
That said, you are welcome to provide any additional supporting information or documentation to the landlord directly to further prove how you will afford the rental amount (if required).