Why should they perform?
Emphasise the things that make your gig special - especially the fact that, by performing, artists will be helping you to raise lots of money for an amazing cause.
As a charity event Oxjam never pays an artist to perform.
Timings and logistics
Bands will want to know the location of the gig, what time they need to arrive, sound check, and the curfew.
Most bands will have 'tech spec' for the equipment they need to perform, which you'll need to check with the venue. If possible ask your bands to share or bring their own gear (e.g. drum kit), and avoid hiring equipment wherever possible.
Make sure to ask if your acts will be happy to promote your event to their fan base via social media. They may even help you to sell advance tickets via their networks.