Actually, it was much earlier (and the GLC was not formed until 1965).
Plans for regeneration of the High Street commenced pre WWII under the aegis of the local authority of the time (Borough of Brentford & Chiswick), and were the proximate cause for its decline from thereon.
The road-widening scheme, with its threat of demolition of existing High Street buildings, meant that owners & occupiers had no incentive to maintain their properties, which consequently fell into decline throughout the war years. Stan Goddard made this point in a newspaper article dated 1950 -
Post war, the Middlesex County Council carried out the plans to some extent, and CPO’d and destroyed whole swathes of High Street businesses – as you have described. The charmless County Parade was built as the town’s new frontage onto the widened road.
Ironically, as an English Heritage report much later observed – if Brentford High Street had been left alone, it would now have rivalled and surpassed neighbours such as Chiswick for quirky and attractive historical character, and become a genuinely desirable destination for shopping & sightseeing.