The T being added in is what? Maybe fifteen years old?
This didn’t sound right to me, I’m sure I heard LGBT more than 15 years ago, so I looked it up. According to Wikipedia LGBT was first used around 1988 but OED says 1992, other sources say it became widespread in either the 90s or 00s (varies by source). So 15 is a lower bound for widespread usage but I don’t think we can get much more specific than “15 to 35 years ago”.
This is true for all public holidays in the UK, there’s a (usually) fixed number of public holidays but the dates are flexible.
They’re also included in the minimum 28 days paid time off too, meaning if you’re a full time worker and have to work on a bank holiday your employer is legally required to offer an extra day off somewhere else instead, either a fixed date or added to your holiday allowance. Conversely, the “extra” day off you get when a monarch keels over may be subtracted from your holiday allowance for the year. This is also why my employer is allowed to follow English bank holidays despite having next to no presence in England; the number is fixed but the dates are not.