MLRS is not built into any of the databases in the ATF or AATF series, but the engine supports the weapon type. It would be simple enough to build the vehicle into any database. See this link:
MLRS is modelled in Battle Group Commander but is not used in any of the sceanrios. In fact almost every combat vehicle used by the British Army is modelled so that scenarios builders can replicate most British orbats. I think you will also find MLRS in Curt's Raging Tiger.