Paying legal fees for a criminal organisation is not (by my reading of the law) an offence. But if it is, retrospective legislation is prohibited in all but a few situations.
There are two issues here:
- Support for an organised crime group
This is an offence contrary to section 45 Serious Crime Act 2015:
(1) A person who participates in the criminal activities of an organised crime group commits an offence.
(2) For this purpose, a person participates in the criminal activities of an organised crime group if the person takes part in any activities that the person knows or reasonably suspects—
(a) are criminal activities of an organised crime group, or
(b) will help an organised crime group to carry on criminal activities
HOWEVER it seems unlikely that "donations to help them afford the legal costs they face after their actions" would meet the emboldened requirements at subsection 2(b) as it does not appear to be supporting "criminal activities." That said, each case will be considered on its own merits.
- Retrospective legislation
Generally, this is prohibited under the European Convention on Human Rights, enacted under Article 7 Human Rights Act 1988:
No punishment without law
1 No one shall be held guilty of any criminal offence on account of any act or omission which did not constitute a criminal offence under national or international law at the time when it was committed. Nor shall a heavier penalty be imposed than the one that was applicable at the time the criminal offence was committed.
2 This Article shall not prejudice the trial and punishment of any person for any act or omission which, at the time when it was committed, was criminal according to the general principles of law recognised by civilised nations.
However, paragraph 2 does allow for retrospective legislation if the need arises. For example the War Crimes Act 1991 retrospectively criminalises murder etc committed in Germany or its occupied territories during the second world war.
Although tagged germany, I have answered according to the LawSE Help Centre: "we expect and encourage answers dealing with other jurisdictions ... please tag your answer using the tag markdown: [tag: some-tag]"