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CRC often sells "eurostyle" (left->front) brakes which don't sell well so get discounted:
Acera M396 (rrp £30), Black £23.49, White £19.99
http://www.chainreactioncycles.com/shimano-acera-m396-disc-brake/rp-prod119954SLX M7000 (rrp £95) Metallic graphite £64.99
http://www.chainreactioncycles.com/shimano-slx-m7000-disc-brake/rp-prod148226These come already bled and usable, but normally don't include the rotor or whatever IS-PM adapter your fork might need.
Shimano brakes often include a hose shortening kit: olive + barb + yellow cutting guide blocks. Nothing stopping you from buying a left-rear brake and shortening the hose (or taping the excess to your frame if you don't want to risk a host mistake turning a shiny new working brake into a dangerous bodge).
Looking for a disc fork + disc brakes + disc guard + left leaver if anyone has it up for grabs. Otherwise any tips on a good fork would be cool. Cheers