You should be able to have the St George's Cross flag, it's in the flags image used for this site, click (5 down 1 across), and also available as a separate gif that's used in forum posts and profile pages, click.
Contact WGT at email@example.com to see if they can help.
If you are a bit of a nerd and want to see what the flag would look like on your post above and you use Chrome or a Chrome based browser, try typing or copy/paste or click/drag this into the browser address bar and hit enter...
java script:(document.querySelector("[title='United Kingdom']").src="https://securewgt-a.akamaihd.net/assets/community/images/wgt/flags/GBR-ENG.gif")()
Ever get the feeling, 'Why bother?".