I have a relative who was serving in British Mandate Palestine, who in 1946 married a local woman in Jaffa. I have the marriage certificate from the GRO, but would there exist any records in Israel, or possibly in British Army records, relating to the marriage? The certificate I have simply lists her as 'Palestinian', and I'd like to know more about her background.
The Israel Genealogy Research Association (https://genealogy.org.il) is in the process of indexing marriage and divorce records from the British Mandate period. You can search their database and see all the information for free, but it requires one to register on their site first. Viewing the actual marriage certificate requires membership in the organization. Keep in mind that this is a multi-year project and not all certificates are indexed yet.
Just a clarification to add to the previous two comments: the Israel Genealogical Society (IGS) and the Israel Genealogy Research Association (IGRA) are two separate genealogical organizations. The latter one, IGRA, is the one who has the more than one million records available on their All Israel Database (the AID - https://genealogy.org.il), and they have already started indexing and posting the Mandate period marriages.
The AID is totally free to use, but you do have to register your e-mail account first, and only dues-paying members can see some of the raw images. The AID is bilingual in Hebrew and English, both the search interface UI and also most of the indexed data fields.