Standard springs are priced from £9.00 to £19.50 + vat, each, depending on the chassis number of the Disco.
H/D springs are around £50 + vat per pair, so not that much difference really.
Shocks, you can go for extended gas shocks like Procomp ES9000 that are about £35 each or go for standard shocks with a 2" drop plate for the rears but you'll need something to relocate the springs as these will come out of the spring hangers on full articulation.
You can secure the spring to teh hanger with jubilee clips but make sure the bottom securing plate is in good condition. This will limit the axle drop to the length of the spring though.
Other than that you can fit dislocation cones, but that's a bit more expense.
I took the prices from
http://www.paddockspares.com/ or try
http://www.mm4x4.co.uk/.