If you chose to use an existing e-mail address you must configure the existing e-mail address to forward incoming mail to the exchange account. The exchange account's email will be your exchange username at(@) exchange.1and1.co.uk such as firstname.lastname@example.org.
When sending mail from your exchange account, it will look as though the mail is coming from the existing e-mail account you typed during setup (such as email@example.com). When someone replies to an e-mail sent from exchange, it will route back to the initial e-mail address (meaning the incoming mail will arrive in the Gmail inbox without a forward). Without setting up a forward to the exchange account, you will have to check your incoming mail from the existing e-mail account entered earlier.