There are grants available, and especially after coronavirus. Many charities award grants to schools to support SEN as well as families. Google for grants and you can sites like: https://www.disability-grants.org/educational-grants-schools.html some of the trusts are currently pausing applications but from looking around these are starting to open up.
There are cheap ways of doing sensory rooms and there are a lot of resources available. Some things could be made through school projects as well as by staff. Check out Pinterest https://www.pinterest.com/marmarhughes/cheap-sensory-room-ideas/
Some things may be simple white tents/umbrellas with lights
Hope that helps, with the other information
Founder @specialiapps, educational apps for children with additional needs