September is one of the most expensive months to cruise but you may be able to pick up a late deal. Transocean Cruises and Louis Cruises both sail out of Tilbury Dock which is close to London. Try your travel agent or internet with either Thomson Cruises or Island. At this time of year you may be restricted on Budget. Good luck, there is always a deal to be got somewhere.
I have just noticed on this web site that if you click on Late Deals on the top bar they are offering Ocean Villiage 2 for September departures for £499. This will be as good as it gets.