The Henley unit is the service head, it just has more than one set of fuses. In this area it would be sealed. The cutout in each flat is often red with a solid link in that is provided so meter fitters have a convenient point of isolation.
Different areas have different ideas on where the customer wiring starts and the DNO wiring ends. The last 2 blocks of flats we did the customers wiring started at the meter that was in each flat. The wiring to each flats cutout (beside the meter) was property of the DNO. The flats were also fed from a multifuse service head and had a cutout in each flat, similar to what is shown in your photos. Both the service fuse and the flats cutout fuses were 80A.
Here, any testing would be limited to the flats consumer unit and final circuits. If there was a fault with any of the wiring feeding any of the flats, or if one of the 80A fuses needed replacing, we would not be allowed to carry out any repairs and would have to call the DNO. Well that's what the Western Power bloke said anyway.