Can I buy using cash from a sale of stock?
Yes, as soon as you have sold stock in your account you can use the cash to re-invest immediately. If you are looking to withdraw the cash however, you would need to wait until the trade fully settles - which usually would be 3 working days.
For ShareDeal certificated accounts, any proceeds will be paid to you directly via cheque or to your bank account. You can then use these proceeds to re-invest if you wish.