Functional ground usually means you want to connect directly to the wire leading to a grounding rod. It's an odd requirement that you have both here, because your neutral is typically bonded to the ground at your service entry, and then probably tied to a grounding rod on top of that (depending how old your service connection is).
It could be they are to ensure that a ground fault can occur. UK codes require a Residual Current Device breaker (we call them Ground Fault Circuit Interruptors, or GFCI in the US). They detect an imbalance in the hot and neutral and cut the power before any lethal shocks can occur. As mentioned here
A new, revised version of Regulation 411.3.3, relating to RCD protection of socket-outlets, forms part of BS 7671:2008+A3:2015 (IET Wiring Regulations Seventeenth Edition), which was published in January 2015 and comes into effect on 1 July. The current and new versions of the regulation are shown side by side below. By comparing the two versions, it can be seen that the new version
- requires RCD protection to be provided for all socket-outlets rated at not more than 20 A, not just those that are for general use by ordinary persons
BS 1363 (the UK standard for wall sockets) is a 13A outlet, so this applies. The other moving part here we need to be concerned with is the step-down transformer that's dropping the voltage from 230V to 5V for the USB portion. Apparently BS 1363 changed in 2016 to include in-plug chargers like this, but within certain size restrictions. This teardown of a UK USB socket notes
The underside of the circuits show a heavy reliance on surface mount components. This is most likely due to the strict standards of the British Standards Institute which define the size and shape of socket outlets (in other words, not much room to play with!). Any device that does not meet the BSI standards is considered a deviation and therefore must be checked by a competent person (which individuals may not want to do).
The circuitry to step the voltage down has to be small to fit the mandated form factor. It could mean this device uses cheaper parts to fit in, which . I should note that it seems that BS 1363-2:2016 also precludes relying on the RCD to prevent shorts, but I could easily see cheaper devices not paying attention to that and doing that anyways. It's also possible they're hedging their bets because a great many parts of the world use 230V standards set by the UK