You can begin trading as soon as you receive your incorporation documents.
You must also legally set up a business bank account for a LTD business. Currently, some high street banks are taking a long time to set up business bank accounts. Make sure that you keep very careful records of all payments in and out of your company whilst you wait.
If you want to start trading immediately, there are companies that can provide you with accounts instantly. They tend to cost more to use but will be a good intermediary solution. These accounts are also ideal for clients that have poor credit ratings. We can help you with this if you need further information.