Maybe every 6 weeks in the winter but not in the summer when its very rare, and even in the winter its normally on a sunday when demand is very low. As for the extra routes all the studies have shown that they wouldn't be economically viable, in fact not even nearly economically viable.
But the demand isn't there from Lewes to London. I often pick up a London Bridge train from Eastbourne in the morning at East Boredom and its often half empty. The same with the return train. And also the time for a Brighton - Lewes - Uckfield - London train was only about 20 mins quicker...