So whats the difference between using a circular saw and a drill whilst
standing on a ladder?

Well, both should not be done. However, there is more potential for problems
with the circular saw. A good drill will have a rotor brake that stops it
dead when dropped. Even spinning it is likely to cause fewer injuries than
an out of control circular saw. However, there is potential for both tools
to snag, throwing you off the ladder to your death or serious injury.

I have been on jobs where I have had no alternative but to use a circular
saw.

But you probably had an alternative to the ladder. According to HSE, a
ladder is not a "work platform". However, there are alternatives that are.

Christian.