Hmm...a bit messy!
There should be an AC cable running from the inverter. We would normally run it to the AC isolator, thru the generation meter and to an MCB either in a dedicated board (as here) or in a spare way in the main board. If the PV generation meter is clocking up correctly, you shouldn't need to touch it. If you do, pay close attention to the wiring inside as it's possible to wire it up the wrong way round and the PV system will work without the meter clocking up.
The top box is a V-Phase voltage optimisation unit. It may have been fitted as part of the PV install. You should find the voltage on many circuits is around 220v (test a plug socket somewhere before you start). It's worth keeping now that the householder has invested in it. The V-Phase company is no longer trading but you'll find the instructions online or maybe the householder has them.
The lowest Wylex garage board should contain a 16A or 20A MCB (assuming it's a regular 4kW or smaller domestic installation). If you're going to upgrade to a metal 17th edition board, you could include the PV circuit on the non-RCD side. This may remove some tails and a Henley block from the equation. You could also move the V-Phase MCB into the new board.