Out the box the latest version of ispell on Nix Packages and built in Emacs does not have UK/British English default. So I started looking on Reddit and Googled ispell and Nix to see where I could go.

I am a strong believer in keeping things simple and with the introduction of 'use-package' and the extra features in 29.1 I was keen to rewrite my config and use as close to vanilla setup as possible. I took the very base of what was needed and started from scratch building outward and exploring built-in packages. My config file is available here: nix-darwin repo

Add this to your init.el:

;; Spellcheck
(require 'ispell)
(setq ispell-program-name "aspell")

(dolist (hook '(text-mode-hook))
  (add-hook hook (lambda () (flyspell-mode 1))))

With the emphasis on declarative, reproducible systems, I've been using Nix for some time now. I have a config for Linux/NixOS outside of work, when at work I use a Macbook so have a Nix Darwin build for that. The addition of aspell with dictionaries was surprisingly simple once I'd worked out how to declare it in my configuration file:

# System Packages
environment.systemPackages = with pkgs; [
  # Applications
  (aspellWithDicts (dicts: with dicts; [ en en-computers en-science ]))
  ispell
  # Other Packages
];