Re: ...accurate for a period of 158 million years...
According to Wikipedia, the UK hasn't made up its mind whether GMT means UT1 or UTC so is there some ambiguity. But in all non-nautical contexts, I would expect GMT to mean UTC.
The relevant legislation is clear - it means GMT. GMT is its own time standard which isn't really surprising given that it predates any of the Universal Time standards as the original world time system. The nearest UT equivalent is UT0 but even that isn't a direct substitute since the precise methods by which they are defined differ - the GMT definition is slightly sloppy for modern standards of precision.
The ambiguity arises from the popular conflation of the GMT to mean UTC, UT1, or whatever else the user wants it to mean. "Basically equivalent for everyday purposes" gets morphed into "exactly the same" or even "equivalent by definition". GMT has no fixed relationship with any of the universal time standards. It's also the kind of incredibly subtle issue that I wouldn't trust Wikipedia at all for - everyone fervently maintains what they believe is correct and precise, even if as a poster here remarked they are actually using a simplification of a simplification.
Confusing matters even more in the UK is that although legally the time is defined as GMT, most precision time sources, such as the speaking clock, MSF, GPS and the BBC time pips all actually transmit UTC.
Biting the hand that feeds IT
