In small (model engineering) quantities try Folkstone Engineering Supplies at http://www.chronos.ltd.uk/.
In rather larger quantities in Kent try <A target="_blank" HREF=http://www.parkersteel.co.uk/>http://www.parkersteel.co.uk/</A>. The delivery charge from Parker Steel is quite high, so it's best to collect.
I can make stainless steel backing plates to your sketch on my CNC punch press for small money
complete with mounting holes and formed to suit hull or deck contours if necessary.