Usernames have to be text.
Ah - ok. Thanks.
I see there is indeed a blue heart in MacOS emojis and symbols, but it’s not the same blue heart.
Yeah, that's an emoji. It's a text character. 💙
From some cursory googling, the unicode consortium (quite sensibly, IMHO) don't permit private symbols to be added to the character set. Which is why we don't have a Coco-Cola emjoi, or an Apple logo
[1], or that Prince thing. The NHS logo would appear to fall foul of the same rule.
[1] Unicode define Private Use Areas where developers are allowed to do what they like with the code points without conflicting with officially assigned characters. There's an Apple logo in there. If you're on an Apple system. Other OSes are free to use it for a CDC character or whatever.