Check your tenancy agreement but normally it states that decorating is the responsibility of the tenant. Often the tenant has to get the landlords permission for the proposed work.
If your landlady is willing to have the decorating go ahead but you can't afford to pay a commercial decorator to do the work there may be a couple of things worth exploring.
Firstly, in some areas there are schemes to support disabled people with such tasks, perhaps providing voluntary labour or getting people on probation to do the work. Not every area has a scheme and they vary from one area to another. To enquire if there is scheme in your area contact your nearest Home Improvement Agency (HIA). If you can't identify your nearest HIA then contact Foundations as they hold a list of HIA's: http://www.findmyhia.org.uk/
Secondly, you could try applying for charitable grants towards the cost of the decorating. On the Scope website there is grant search tool:
Jean Merrilees BSc MRCOT
You can read more of my posts at: https://community.scope.org.uk/categories/ask-an-occupational-therapist