Do all parts of the car have to last 6 years by the sale of goods act? I'm pretty sure they don't, and the onus is on the consumer to prove a manufacturing defect after 6 months.
The 6 year term is a maximum and is based on what it is, how much it costs and how much wear and tear/environmental conditions the item it is subjected to/expected to be subjected to.
Expect a TV sitting on a TV stand or screwed to a wall to last 6 years, expect a car engine or gearbox to last 6 years. I doubt any small claims court in the country would expect a clutch to last 6 years unless you could prove you'd never ridden it at junctions or had an independent engineers report following inspection that strongly implied a manufacturing defect in their professional opinion, no one would expect shocks or brake disks/pads to last 6 years either.
Saying all that, i'd expect shocks to last the 3 years of the standard warranty, especially if the brand of shocks fitted to the car come with a 3 year manufacturer's warranty when bought privately as replacements.