Sounds like you are on the right track. Before spending a lot on a big brake kit, make sure the basics are as good as they can be: check calipers for any sticking, give the system a proper fluid change and bleed, and try quality pads and discs first. Braided brake lines will usually sharpen pedal feel and consistency more than they will reduce ultimate stopping distance, so they are a sensible, relatively inexpensive upgrade to try alongside new pads and discs. If you do look at aftermarket big brake kits, the sites mentioned are useful places to start: RHDJapan, Blackhawk Japan, NeoTech in Korea, and sellers like GodSpeed who list Blitz and Endless options. Some kits I have seen include calipers, discs, pads, brackets and braided lines for around 1300 quid, but be careful about fitment. You will likely need specific disc sizes and carrier brackets, check wheel clearance and ABS sensor locations, and think about the extra unsprung weight and heat management. Good idea to get the service to report back on the calipers and lines tomorrow, and please let us know what they find and what you end up trying.