TB, try
this in which I've tried to show a worked example. (Supporting info
here).
Also, to be pedantic, there is actually no legal requirement to inform HMRC and Immigration on your
outbound leg, but many people do. It's no real hardship, especially for a day trip as you'll be sending the GAR for the Return leg anyway.
Unfortunately the GAR needs to go separately to Immigration - the NCU don't pass it on.
As well as the options given already, opening your flight plan can be done with London Information (freq on chart).