Unfortunately, the Plod never confirm receipt electronically. You get a phone call with a "permission number" (they like to pretend that you need a "permission"), and this can be any number of hours later.
Actually that is a bit of a red herring. You do not need permission or a permission number. You are only obliged to inform the constabulary of a trip. You are not obliged to make sure they got it or act on it. Therefore a simple fax receipt is all you need if you are ever required to prove that you made the notification.
The system is very simple, you make the notification on the GAR form and send it off, this can be fax post or email. You go on your trip. If they pull you on return the onus is actually on them to provide the proof not you. We have a nice system in the UK that says you are innocent until proven guilty.
If you get hassle from SB as I did once going back into Cranfield from Guernsey then complain to their superiors and point out how the law actually stands.
A website that does this is a simple enough thing to set up, the problem is the donkey work in the background getting and keeping current all of the phone numbers and emails.
I will raise it at the AOPA MWG and see if it is something that they would look at again.