Hastings Diesels will be your main operator for tours coming from Sussex, with their base in Hastings.
They only run about 4 or 5 tours a year though.
UK railtours are more frequent, normally from one of the London terminals, typically diesel tours, mostly appealing to people who want to visit the destination and a little extra leg tacked on bit for the track bashers.
Railway touring company run quite a lot of steam tours from the London terminals , typically to more touristy destinations. Steam Dreams also run some , for about £50 more than what RTC and UK railtours would charge