The speed index must match or exceed the maximum speed of the vehicle and not the national max speed limit of 70 mph. The vehicle's maximum top speed can usually be found within the vehicle's manual. For example, a tyre speed index over L (75 mph) is suitable for the purposes of driving under the national speed limit, if your vehicle's top speed is 150 mph, than you would need tyres with a W (168 mph) rating or higher.
For load indexes you must match or exceed the load specified by the vehicle manufacturer as found usually within the vehicle's manual.
Tyres with duel load index markings for example 265/75 R16 112/109 R can carry the load indicated by the load index "112" (for single use, 2 tyres per axle) and the load index "109" (for dual use, 4 tyres per axle).