I assume the ticket you have found is a saver return with a railcard - if so that's the cheapest you will get it on a direct routing. There are no advance purchase tickets on the Worcester line.
If you are prepared for a slightly longer journey try Deal to Cheltenham (from �8.60 advance single each way with a railcard) and then get a local train to Worcester (c�4 standard single each way with a railcard)
Or try Deal to Birmingham then a local train - haven't looked this one up though. Might be a bit quicker and more frequent than Cheltenham