1. Looks like an Aqualisa "Aquavalve 609".
2. I don't believe you can replace the knob with a lever. The knob contains the maximum temperature limiting mechanism, which wouldn't fit into a lever.
3. I think you have two options:
3.1 Replace the whole shower valve. This would require some excavation of the wall, and probably a few new tiles.
3.2 Get a brass (real brass, not "brass effect" or "plated") angle bracket. Cut one side down to a bit under the depth of the knob. File the other side to make it look a bit better and lose the sharp corners. Fix the short side to the knob with a stainless steel jubilee clip of appropriate size. Don't overtighten.
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