the mains uk voltage is supposed to be how many volts?

The mains voltage in the UK is nominally 230 volts, with a tolerance of +10% / -6%. This means the voltage at the supply point can legally range from about 216.2 V to 253.0 V, though actual voltage entering a building can vary further due to local drop and load conditions. In practice, many references still colloquially refer to “240 V” historically, but the official nominal is 230 V with the stated tolerance. If you’re checking equipment compatibility, design margins typically assume the 216–253 V range at the supply, with downstream voltage drops potentially reducing voltage further in some circuits.