There is no such thing as the best time during the day to trade energy. The market is driven by fundamental, technical and psychological drivers and indicators. The best time to trade would be according to the signals provided by these indicators and in line with an agreed strategy.
Historically, energy contracts often fall off as they approach their expiry date due to an eroding risk premium, but this isn't always the case. In addition, Ofgem recently introduced a liquidity window, meaning that there is more energy market transparency and liquidity in the electricity market between 10.30 am and 11.30 am and between 15:30 pm and 16:30 pm from Monday to Friday, which is favourable to market participants.
While liquidity and transparency is increased during these time windows, that does not mean it is the best time to buy or sell energy.