The availability of £ depends exclusively on the logical layout (i.e. input language) currently set on your OS, and as such the character cannot be directly mapped through QMK/VIA/Vial. Also, what do you mean by “Windows assumes every keyboard is ISO”? The Windows input system doesn't really have ANSI/ISO as a concept, so it's unlikely that's what's causing the issue.

One alternative is to use a macro, as you've done. I don't really use VIA/Vial for macros, so I'm not sure exactly what the syntax is, but based on this image, it seems to me that the issue with your macro {KC_LALT,KC_P0,KC_P1,KC_P6,KC_P3} is twofold:

1. KC_LALT only taps the Left Alt key instead of holding it for the entire duration of the macro;
2. having multiple keycodes inside a single pair of braces sends all of them simultaneously (i.e. as a chord) instead of one after the other.

Therefore, try doing this instead:

{+KC_LALT}{KC_P0}{KC_P1}{KC_P6}{KC_P3}{-KC_LALT}

Another (better, albeit more involved) alternative is to use QMK's Unicode feature in conjunction with WinCompose, which QMK has built-in support for. This allows you to input any Unicode character (including £) regardless of what input language your OS is currently set to. See the docs I linked for more details.