Don't forget you'll need new pedals too.
£100 budget should get you something decent, second hand with a sealed BB, maybe Primo hollowbites, profiles or twin pinch redline flites.
The other day, either Winstanly's or chain reaction had Blank cromo 3pc cranks, with mid BB for £20 posted, (reduced from mid £50's) you'll need a US BB, but you can sell the mid to offset the cost., colours were limited, but i've had some and they are ok quality.
MM