This point made me do far too much digging into trains XD
BR was privatised between 1993-1997 during which time the colours were often kept at a stock green, similar to the ones seen in the first film, but that doesn't prove anything either way because First Great Western also had green trains in the 2000s operating out of Kings Cross. Moar ambiguity.
If only an official date had been given, my anal retentive streak wouldn't frustrate me so >>;