Hi Albaba,
I have a child with severe SEN so I am only to happy to offer some ideas 
Which age range are you looking to assist with SEN? There are many different settings to work with children with special needs, not just schools.
However, starting with schools...some SEN schools are from 4 to 18. There are opportunities to work with SEN but you would need to be prepared to assist on physical aspects of the job (toileting, etc). Someone I know was a TA in a SEN school and is now training as a Disabilities Nurse.
If you are looking for SEN in a mainstream setting, Its is quite right about the qualifications and excellent advice.
Have you thought of working as a play worker with SEN at the local children's centre or as a portage worker?
There are also specialist respite centres (do have a look in your local area), which you could consider.
Alternatively, there are SEN pre-school charities or SEN charities, such as CEREBRA.
As you can see from above, there is a lot of scope for you are considering and if you want to work with special needs children, there is a lot of opportunity for you to find the right fit.
Best of luck!