Not paying up front is an absolute.
I think people who routinely post this as a throwaway line, however well-intentioned, are doing noobie pilot visitors to the site a disservice and actually giving them poor and potentially costly advice. Here is a much better way of saying the same thing:
Small flying schools have a long (and sometimes sordid) history of pocketing your cash and then going bust, so don't pay upfront unless you can get full third party advance payment protection on your money before you hand it over.
There are cost savings to be had by paying up front and there are (FCA regulated) companies that offer advance payment insurance. I have no idea of the premiums and t&c, but Google and a calculator should easily enable anyone to work out the cost/benefit.
Futhermore, credit card purchases are protected up to £30K, under the consumer credit act, (you don't even have to pay the full amount on your credit card, for the full amount to be covered!!) again, I don't know the detail and whether flying training is covered but it will be short work with Google and maybe a phone call to your CC company find out. And obviously you can pay of the CC bill the next day, so there is zero need to incur interest charges.