Hi Tracy,
If you manually record your expenses and then later connect your bank feed, the system will not automatically know they are the same transactions. This means if you import a bank statement that includes those same expenses, they will appear on your bank explain screen as items needing to be processed. To avoid duplicates, you would simply need to match the imported bank transaction to the manual expense you already created by clicking Reconcile against the relevant transaction.
Regarding your question about HMRC, you do not strictly need a connected bank account to submit your returns. HMRC requires digital records of your income and expenses; how those records are created, whether by manual entry or bank feed, is up to you, provided they are accurate and complete.
And yes, when you link a bank feed, you can specify the import start date.
Tommy