Flying once a month does make it almost impossible to actually get a licence.
The time gap means that you forget the vast majority of what you have learned in each lesson. That is a fact I'm afraid.
Throw in the general UK weather and you can end up missing half the booked lessons which means that once a month becomes 6 lessons a year. Getting a licence...No chance.
The more often you fly the easier you'll find it. If money is tight, then save up and do lessons in a block. Stringing it out is wasting your FI's time and your money.
I define progress as not having to repeat lessons or even part of lessons. There is more to it than simply learning checks and procedures, you really only learn properly by doing things, not reading them in a book. It helps of course and you are doing the right thing by being prepared, but speaking as an FI who's been teaching for a while, then once a month just isn't enough I'm afraid.