That was a 6% drop. Perhaps the other 4% kicks in at maximum rated load when leakage inductance and transformer winding resistance volt drop take their toll?
It sounds to me like it's a basic, if BFO[1], autotransformer. It also sounds like it's intended to be installed between the meter and CU to reduce the whole house supply, presumably to compensate for a PSU line voltage that's consistently on the high side of the nominal 240v used in the UK.
If it included automatic voltage adjustment to compensate for variations in supply voltage, it would make sense. If it's just an autotransformer, prewired to provide a fixed 10% reduction as your post implies, it doesn't make much sense since the proper solution is to complain to your PSU about the overvoltage and arrange for them to adjust the supply back to the nominal 240v.
However, 251v is within the +10% upper limit for a notional (harmonised)
230v supply upon which all domestic kit, excluding incandescent lamps, is optimised. If the supply never exceeds 253vac using an accurate voltmeter, then it's within tolerance and unless the supply regularly exceeds this, quite frankly, I can't really see the point of inserting such a BFO autotransformer between the meter and the CU.If this has been installed in the mistaken belief that it will reduce electrical consumption by household appliances, as almost everyone else has pointed out, it won't. Indeed the additional losses, even if quite tiny, will increase consumption negating any small gain made with incandescent lamps which will become noticeably dimmer, yet at the same time may last twice as long as normal (so, not all bad news if you don't minder the dimmer look).
If the average line voltage is above nominal, the 10% reduction shouldn't be a problem to white goods appliances which use electric motors, especially if they're modern 'Harmonised' appliances. However, too little voltage can cause motors to overheat and burn out, especially during startup surges with compressor loads such as fridges and freezers and, to a lesser extent, washing machine drum motors.
Kit that uses smpsus such as TV sets, desktop computers and the like, will automatically draw exactly the same power, typically over a supply voltage that ranges from a low of 186vac to a high of 265vac. Some of this kit may, like universal mains voltage laptop chargers and other battery chargers, function over the range of 90vac to the 265vac limit, drawing pretty much an almost constant power level according to demand from their electronic loads.
Most appliances that rely on heating elements for their function will simply use a longer duty cycle controlled by a thermostat. So, again, no net saving in energy consumption there. Even electric kettles will simply take longer to boil before their anti-boil dry sensor conveniently switches the kettle off as a kindness to the user. The more protracted heating up time will allow a little more heat to escape increasing the energy consumption slightly as it will in the case of every other appliance that uses energy for heating.
In short, if this "Voltage Optimiser" has been sold to your customer on the basis of reducing electrical consumption, then it has been sold as a "Snake Oil Solution" to a non-existent problem.
[1] Assuming a 10% drop on a 250v 100A supply, you'd need a 2.5KVA rated autotransformer which would weigh in, afaicr, at around 25 to 30Kg which in a domestic installation is one Big F*ck Off autotransformer! :-)