I would bow to Dave's greater experience.
I shouldn't if I were you! I agree the standing gull does look rather like Yellow-legged Gull and I don't have enough competence to rule that out. I'm just not quite sure that it's safe to judge the mantle/upperwing colour as being as pale as it seems. Personally I find other features like head/body shape hard to use reliably in the field, and nigh on impossible in a single photo. I don't know if there's anything useful in the primary pattern that someone better than me could use to clinch it?