@Sean *******
as long as you meet all the requirements your visa application will get approved, sometimes they ask for additional documents, you need to print the E-visa and carry it in your passport, it has an issue date and a use by date, you can enter Thailand anytime within that date to activate the visa, and you get stamped in for 90 days, the London embassy are very quick at issuing e-visas, people report getting them usually within a week, so a month before would be okay, I've send an information link in another comment, that explains the way to apply on the E-visa site.