UKBA say that if you send your application form and application fee by post, the date of application is the date of posting. It's decided by the postmark. If the postmark is illegible or missing, the date of postage is taken as being at least 1 day before it is received by UKBA.
If you send your application form and application fee by courier, the date of application is the date when it is delivered to the UKBA payment handling service.
Applying in person at PEO obviously means the application date is the date when you attend and make payment.
I have no experience of likely outcomes if the application is received more than 28 days in advance. Your guess is as good as mine. Probably a day or two is OK. I just don't know 100% sure. I never think it's worth a risk to go against UKBA procedures as no-one knows quite what to do.
Mark, if the FLR(M) application is made on May 15 (Friday) or May 18 (Monday) that would be fine.
If the IELTS certificate is valid at that date of application then it's already compliant and good. So yes, Mark that's perfectly OK.
Good luck to your wife for her upcoming test![]()