There are two possibilities for people who have genuine reasons - deferment and exemption. Changes in the law have meant that very few categories of people are exempt these days. Magistrates, for example, who already give a substantial amount of public service for no pay (minimum of 13 days per year, usually quite a bit more) can still be called for jury service.
If you were called and could convince them that it would significantly damage your business to serve at a particular time, I expect you would be able to defer to a different time. I doubt they would exempt you. Once deferred, they would probably expect you to go and make arrangements to cover whatever business obligations you have for when you are next due to serve. I guess the argument would be that juries should be as representative of the adult population as possible, so it wouldn't be desirable to allow every self employed person to be excused. Just my thinking on the logic. I work within the legal system, but I'm no expert on how the jury selection system actually operates.