If it were me I would leave all the non-freesat channels loaded but I would move the ones your interested in up to the begining of the channel list.
While in non-freesat mode press 'Menu' > Edit channels > Edit Channel List > 'OK'... now scroll through the list pressing the up/down cursor key or a page at a time by pressing the left/right cursor key. Locate the channel and click 'OK' to select the channel, you can then locate the next channel of your choice and press 'OK' to select this one. Repeat as required. Now press the green button and the drag the selected channels up to the top of the channel list by using the same method utilising the up/down left/right buttons. Once in place press 'OK' to drop the channels in place.. Press exit and save changes when prompted.
If you really want to delete the channels then repeat the above but press the red button instead of the green one and just confirm the selection by pressing 'OK' instead of dragging them around.