find the File/Edit/View.... menu at the top of Thunderbird, if you don't see this menu press the Alt key to reveal it.
Click on "tools" then "options" and you will see the following window
Select Composition (highlighted blue in above pic)
click on the text "download more dictionaries"
this will open a new tab in Thunderbird. Scroll down and fine English (British) in the list then click on the download dictionary link as below
You will then get this screen
Click on the green + Add to Thunderbird button
you will get the following dialogue box, wit a few seconds for the install button t become available and click it
You should get a message saying installed successfully as below
Go back into the tools, options, composition dialogue
Go back into the tools, options, composition dialogue and check/change the language: drop down so it shows English British and click OK
You now have a British dictionary installed and set.