You wont get one for £40 from any decent brand. Avid Juicy 3's are probably the best budget hydro disc at £50-60, but as above, you'll lose the cut-out.
If Wisper sell their DaaHub cut-out switches they might work for you. Best to check with Wisper.
I am not sure which bike you have so don't know whether or not they will fit. Have a chat with Steve in support. We stock Tectro electric bike hydraulics with cut outs but they are over £100 the set back and front wheels.