i wasn't ever on call when i did psychiatry - haven't done palliative care, so couldn't say. generally speaking we don't do many on-calls at GKT (i had 4 over the whole of my third year, each time i was heavily encouraged to go home!), so you should be ok. i can imagine you'll be free for most, if not all, your weekends.
if you want to do psych, try to do it in the Maudsley/institute of psychiatry - i had a fantastic learning experience there.
because its a busy teaching hospital/s you would probably have alot of control over your own time, and how you choose to spend it. you could make your elective as laid back, or as busy as you want it to be.
hope that helps!