Maintenance loan for open university???Watch
There's some good news, some bad news, and then some more good news in answer to your query.
The first good news is that you probably won't need a maintenance loan for text books. For most modules, the OU provides all of the materials required. Many are developed in-house by the OU (the vast majority of module resources), some are co-developed by them (such as BBC/OU DVD resources), and sometimes they even send third-party materials. (I got a copy of Isaac Asimov's science fiction classic I, Robot for one of my current modules.) There are some modules for which this isn't the case, however, such as literary works for consideration.
The bad news is that there is currently no maintenance loan available for the Open University. You can receive financial loan assistance toward your tuition, but nothing beyond that.
The second good news is that this is tentatively scheduled to change. You may hear many people misquote 2018/2019 as the target date for part-time maintenance loans to be offered. This was the target date given for non-distance learning maintenance. Originally, the Open University and other distance learning institutions were not going to be considered for the part-time maintenance loan scheme. After consultation, however, the current plan is to bring distance learning institutions into the programme a year later, for the 2019/2020 academic year. As you're studying a law-related degree, you may get a kick out of going over a March 2017 consultation response in which this revelation was communicated.
(Keep in mind, however, that March was ages ago in political terms, so who knows if any of this will ever be acted upon. It seems an easy win for any government to pursue, though.)