Cheapest options might be 10-speed bar end, those MTB-pull drop bar Tektro levers, and a 9-speed MTB mech. Could be done for well under £50 if you are patient.
Or, you could possibly get a friction shifter and keep your mech, if you can tolerate friction shifting on 10-speed.
Hmm so I can't use the cassette and deore rear mech (10sp) with drop bar shifters and road levers won't work with my discs...
So the only way is a new road disc setup and a road cassette, shifters and derailer?