I had a look at what they say about battery storage systems and found this "gem"  that tells a different story as it shows their view of the energy storage market.

To choose the right battery storage system for your home, you will first need to consider how much energy you want your system to be able to store, and how much energy it can deliver at once. For 2025, the maximum power the battery can accept when charging, for an MCS certified battery storage system, has been 5.95kW.

In 2025, the average cost for a battery storage installation is just over £8,900.

So some consumers are taking this path and installing a £9k battery system with max 6kw of charging power. The website does not mention the average capacity of the 2025 MCS certified battery storage. 

I was given an estimate of £3.2k for MCS and labour for my 2025 installation. I wonder what storage capacity did a battery system with a 6kw inverter and costing £5.8k have? And what ROI could that £9k investment attain in a domestic setting?