Lucky Boy !
You are very lucky to have two of the best flying training providers on your doorstep, I'm sure that pay as you go would be an option at both White Waltham and Booker.
The most important thing at the stage of your training is the quality, both of the establishments you mention will provide that quality at a price that is not far above that of some others mentioned above.
The other thing that I always say is don't pay up front but these two companies are the exception to the rule, they have a long history of being financially stable, even so it is worthwhile only giving them enough money to secure a reasonable discount.