Will October/November harvest time (in a UK Greenhouse) hurt my plant or decrease yield etc?

Hello. I faced sort of this problem too. My solution was to put a tent in the greenhouse. In the early spring days It was only open for about 6 hours but the light came on early and I turned in on when I closed up the tent and the neibours weren't bothered by the light. Even if you don't use a light you can still control when the plants start flowering by opening the tent at 8 am and closing it at 8 pm. I put my plants into flowering on June 1st and will harvest Aug 15 after 10 weeks of flowering under the strongest mid summer light, not the weak mid fall light. I know it's an extra expense so maybe consider it for next year so you can control flowering times better. Chuck.