There are some pretty verbose rules regarding recording of calls in the UK (& Europe too). If you want some bed-time reading, then have a look at:
http://www.netlawman.co.uk/acts/telecommunications-lawful-business-practice-interception-of-communications-regulations-2000.php
I did work for Racal's voice recording division at one time, and it was our understanding that there was an obligation on anybody recording calls, to ensure that both parties in the telephone call, knew that the recording was taking place. However, there was no proscribed way to do this! ... An in-queue announcement is one way, but, you may choose to simply write a phrase on your marketing material and advertising, something like ... "All our calls are monitored for training purposes ..." According to the letter of the law, that appears to be enough! Note however, that it is just as important to tell your employees that you are recording the call - and it was often suggested that a way of doing this was by inserting a clause in the employees contract of employment.
Other than that ... I agree with comments above, that you'll only need one voice recorded message, but you will need a 3CX DR entry for every DDI number, with a short e.g. 1 sec', timeout and divert to the matching extension or group.