hahah yeah...i was about to say the same. and add a little too..
For self employment accounts, the profits are calculated for you by HMRC when you submit your return. So in reality, self employment accounts are super duper simple for a chartered accountant because they actually don't have to do anything at all all they do is check your figures are correct and submit them so in reality its still just a book keeping exercise as HMRC do it for them electronically.
Ltd company accounts is much more complex, because you could be running a directors loan account, or need to add journal entries, there is also tracking things like NI and pension regulation and all this falls on the responsibility of the company to calculate it correctly, HMRC don't do any calculations for you, you have to do it yourself and get it right which is where a chartered accountant comes into play fully.
When pay my corporation tax for example, i don't get a BILL like self assessment. I just pay whats owed to HMRC according to our accounts and calculations which are open to public scrutiny.