This one is easy!
- In Thunderbird, if you don't see the menu, press the ALT key on your keyboard, and it'll appear
- Go to the Tools menu and choose Add-ons to open the add-ons tab
- Type "british english" and press Enter
- You'll see a list of optional add-ons and dates - look for the British dictionary with the latest date (at the time of writing this is dated 1/11/14)
- Click the appropriate Install button on the right
- Close the Tab at the top so that you go back to the Inbox and that's it
You should now have spell-check enabled as you type, and also you can enable the option to check spelling when you hit Send as follows:
- In Thunderbird, press ALT to get the menu to appear, if it isn't already showing
- Go to tools then options
- Click on Composition on the top
- Click on the Spelling tab underneath
- Click on the tickbox next to "check spelling before sending" as circled in red here
- Click OK