I would go along with booking 2 separate hour lessons and allow yourself a reasonable interval between so you can relax - maybe one in the morning and another after lunch, it is worth trying 3 lessons in the day, I did my PPL on a flying scholarship and 3 trips a day were the norm, but try it and see how you go - I did do 4 hours one day and that was definitely too much. As far as the wx getting worse, I think you will find the best weather in August and early September (It is always bad for Wimbledon and why do you think Glastonbury is known for its mud!) and it may be worth looking at taking a bit of holiday at that time and getting some intensive flying in then.
