Take the front calipers off and clean them up, make sure the pistons are free to move and push all of them back. Fit new pads on the front, grin & bear it on the cost and buy a good name like Ferodo DS2500 or Performance Friction. K-Sport UK can sell you a set, or if you can spend a lot of time researching part numbers etc on the internet you might be able to find the right pads cheaper elsewhere, but they won't specifically be advertised as fitting your calipers, it'll be something like AP Racing 6-piston calipers. I did this in 2012, took ages. Should have written down the part numbers for the future
Check your front brake rotors aren't lipped, I hope you don't need new discs as they are also terribly expensive, I think something like £400 for a pair of rotors, these have to be from K-Sport I think as I couldn't find any cross-compatibility when I checked for myself.
On the back axle, again check the discs, if they are lipped or rusty or scored then replace them, also fit a new set of pads, I would just go for standard pads.
I would also fully replace the brake fluid, keep bleeding it out of one caliper until you see fresh fluid come through (I recommend using ATE Superblue as it is visibly blue in colour and is great quality fluid). Then go around the other 3 calipers and bleed them until the blue stuff starts coming through. Make sure you keep topping up the reservoir as you do this to prevent air being drawn in.
Thats it really - that will get your brakes to be as good as they can be, if they are still not right then I would say you have a problem somewhere.