@Poppyp I’m really so hear about your brother. It sounds really difficult not being able to follow his wishes. I was wondering if you’ve contacted Citizens Advice to find out some information about what support you’re entitled to? You can find a bit more information about how to get in touch with them over on their website - https://www.citizensadvice.org.uk/
You could also try reaching out to Cruse, they have a helpline and email support if you wanted to get in touch to ask them a few questions about what you’ve shared here. They are experts and will be able to provide you with the information you’re looking for - https://www.cruse.org.uk/how-can-cruse-support-me
You may also find this topic on the community useful to read through as other members have shared their experiences arranging funerals and the costs involved - How did you cope with funeral costs?
I hope these are helpful, keep us updated with how it goes 