I have a 2008 Range Rover Sport 2.7 V6
The Vehicle is fitted with 20 inch wheels, with a 275/ 40 profile 20 inch Tyre. Which is no use for off road or even on the road as there is more potholes than Tar. I'm looking for much smaller wheels . I have 360 ml Disks and 4 pot Brembo brakes. Does anyone know what smaller wheels I could use, would like a 60 profile tyre.

 

If you have Brembo Brakes, you will have tough luck fitting smaller wheels, It is not even about outer wheels size as much as about spoke design. I'd assume that 19' wheels can be installed but again, spokes must be designed in such way not to interfere with a caliper. I bought 20' wheels from full size RR and they had locked up on calipers.
I run on 275x45x20 with no issues. so if find 19' wheels, you should be able to fit 55 without any issues.
Look at the spare tire, I think spare tire Rim is 19', so if you get 3 more spare tires, you could use those rims to throw beefier tires you want