A rationale is presented for the use of a relatively low-altitude (15km) Rayleigh Laser Guide Star to provide partial adaptive optics correction across a large fraction of the sky on the 4.2m William Herschel Telescope. The scientific motivation in relation to the available instrumentation suite is discussed and supported by model performance calculations, based on observed atmospheric turbulence distributions at the site. The proposed implementation takes the form of a laser system, beam diagnostics, tip-tilt mirror and beacon launch telescope, together with a range-gated wavefront sensor and processing system. It is designed to operate in conjunction with the telescope"s existing facility-class natural guide star AO system, NAOMI. Aspects of the proposed implementation are described as well as the technical features related to the system model and the error budget. In a separate paper the NAOMI AO system itself is presented. Other papers describe a demonstrator system and preliminary Rayleigh beacon wavefront sensing measurements at the site.