If your happy with things why change them?
I would try and stick with the same instructor if all seems to be going well, a change or a couple of flights with a different instructor is not a bad thing though, best done later in the PPL when your happy at the controls.
The PPL exams require a bit of work, I would guess approx. 20 hours of self study on each of the 7 subjects. I would recommend signing up to the Met office web site, get the aviation weather for the airfields near you on a daily basis TAF, METAR, spot wind etc. You will quickly pick it up, far more interesting than the book although you will need to look through it.
Some of the items which are examined you will never use and some like altimetry which does not seem to features in the exams so much is very important.
I found reading about the flight computer very painful, a chat with an instructor and one evening on nav later I was very happy with it. The key with it practise.
I would not cry off due to the weather every week (it will improve soon!), turn up anyway and make use of your instructor, there is always things you can go through, if the instructors not in the mood chat to PPL's you will pick lots of useful stuff up. (Some rubbish as well, you will soon be able to separate the useful from the rubbish)
May sound obvious but always be ready for your lesson, relax and enjoy! Don't try to rush it let it sink in! You will be better and save time and money in the long run.
Good luck with it..................