Eden Brown is a great supply agency. The staff are very helpful and welcoming, in my experience they have been there for me through any time I have approached them for support or just general advice within my school placements.
The only down side is the pay, however this goes with the job (supply TA) itself being a low paid job across the board, rather than being a fault of the company - for example, you will not get paid in school holidays.
Additionally, job security does not apply to supply work in most situations due to the nature of the work (this can vary). However, the staff are great and have kept me updated 100% of the way in letting me know of any changes or upcoming work.