Would Apple (UK) change a part with another for you?
I'd be very surprised if they would - there's no standard Apple product to replace an optical drive with an SSD. 

You'd either need to do it yourself using any one of the third-party optical bay solutions (see this question) or have a third party hardware repair firm do it for you. In both cases, however, you'd be voiding your Apple warranty as the optical drive is not a user serviceable part.