As stated above, you can move it -3/+14 days and can move to other London airports. Importantly, you don’t have to make up your mind now. You can wait up until the hour before whatever flight it’s proposing. You’ve got a fully flexible ticket in many ways. You can also cancel for a full refund.
I’m surprised you are able to move it to tomorrow though. Are you sure you can do that for no fee or fare difference?