I have to agree that the pricing structure looks simple, but one should be aware that it is a 'sprat to catch a mackerel'.
It's perfectly fine is you want to make just a few calls or texts, or even to use a little data, but if your requirements grow bigger than that, beware!
For instance, you will only get
500 MB of data for a £5 Add On using this tariff, whereas you get
2 GB for £5 if you have an earlier tariff like 3Pay.
(2 GB for £5 also used to be available on the
Flat 12 tariff, but seems to have disappeared now.)