Not entirely sure what you mean by LABC, but I suspect referring to the commercial entity LABC Ltd, that is a trading name of District Surveyors Ltd and acts as a membership organisation for local authority building control teams. They are nothing to do with regulations and are free to make any recommendations and guidance they like as to how compliance with regulations may be achieved, as do the Approved Documents
The 2022 changes to Part L did not make “300mm mineral wool” the regulation. The regulation/guidance is performance-based: for relevant existing roof/loft thermal-element work, the improved roof U-value is 0.16 W/m²K. Depending on product lambda, installation, joists, existing insulation and detailing, that may be achieved with around 250–300mm of mineral wool. 300mm may be a conservative installer standard, but it is not the regulation itself, so please don’t mislead by implying that it has to be 300 because LABC said so (especially when labcfrontdoor says 270) as it’ll just generate confusion