Basic uncapped is offered as a shaped service. The service is typically unshaped as much as possible as standard, however the more peak time usage that passes through the account, the heavier the shaping that will be applied by means of deprioritisation of certain protocols, starting with download protocols. You can ensure that you are not affected by this shaping, by keeping all large downloads scheduled to run from midnight to 6am (off-peak), and ensuring that no streaming services that are bandwidth intensive are left on for excessive hours in a day.
Your shaping is determined by way of a star rating system that looks at usage behaviour over a 10 day rolling window, so should your account be affected, you can always limit peak data throughput temporarily to revert the account back to a less affected, or completely unaffected state. Shaping is applied to ensure that costs are kept as low as possible, which makes this account accessible for users of a low demand nature.