Backtesting uses the current spread for GBPUSD at whatever time you backtest. On weekends, spreads are often substantially inflated when the brokers close. That's why backtests on weekends do not reflect normal weekday spread. If you want to backtest on weekends in the future, try to login in without a password before your broker closes on Friday. This will disconnect your account and freeze the normal spread for the weekend, or until you login again. Hope this helps. Cheers.
That makes a lot of sense! Thanks a lot for your answer.