IDS (or InterDictor/Strike) is the manufacturer's descriptor based on the role of the variant. It simply describes the strike variant, to differentiate it from the ADV (Air Defense Variant) Interceptor variant.
Designated GR.1/GR.4 in RAF service, unique features notwithstanding, they are still considered to be IDS aircraft, as that is the role for which they were built. (perhaps less necessary now, following the retirement of the RAF ADV's)