Meters in British installations are still sealed pretty much the same way as they were 50 years ago -- Just a wire and seal passed through the bolt head which holds the terminal cover in place.

Modern installations are gradually moving the meters into cabinets outside in place of the traditional indoor location, but the cabinets themselves all use the same triangular-key lock mechanism, so there's no extra security in that respect.

An older indoor installation in a hallway cupboard:

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A new installation in an outdoor cabinet:

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