What kind of prison terms could they expect in the UK?
I would say life. The Terrorism Act 2006 says:
5 Preparation of terrorist acts
(1) A person commits an offence if, with the intention of-
(a) committing acts of terrorism, or
(b) assisting another to commit such acts, he engages in any conduct in preparation for giving effect to his intention.
(2) It is irrelevant for the purposes of subsection (1) whether the intention and preparations relate to one or more particular acts of terrorism, acts of terrorism of a particular description or acts of terrorism generally.
(3) A person guilty of an offence under this section shall be liable, on conviction on indictment, to imprisonment for life.