> Obviously even UTC for a location varies depending on the time of year.

The offset from UTC for a location varies depending on time of year but UTC definitionally has a zero offset throughout the year.

If you’re in the Europe/London time zone your time is equal to GMT/UTC (offset zero) for half the year and BST (offset +1) for the other half.

In other words we have two different types here: Timezones based on location where the UTC offset varies, and the UTC offset itself (like +0100/BST or +0000/GMT/Z.)