Extract issue..

[micheus]

Ok, I got your point for the Circularise. Yeah, the [LMB] option doesn't use any reference as start point, so it thinks about each edge ring. It's easy look to a sphere and think "why it do that so complicated?", but the code doesn't have eyes.
That is why you need to know that must of the Wings3d commands have a different action for each of the three mouse's button (LMB, MMB and RMB). Always check the Information line to know about them.

So, you still can use Circularise, but you need to use the [MMB] option and then you can set references for the command be applied. It's very flexible in this case:
- pick the center point [1];
- pick a plane which the rotation axis will lay [2];
- pick the reference point that define the stable ray [3];
Please, for each edge ring you will pick its respective vertex shown in the third image [3].

For the UFO, I still think you could just use Extract/Inverse/Intrude for the extracted piece (mainly because you did that using normal direction) and - still using the initial selection - use Extrude in negative direction, but using the same D value (thickness) you used for Intrude.
Of course, you must store the initial selection and that way this workflow make you get this shape without to need select anything else by hand - it's like a all-at-once action to get it done:

Doesn't it look similar to your UFO? (I hate this emoji)