As I recall what people wanted was a simple web page that they could complete and hit send
It's been around for a few years - homebriefing.com
But it has a huge massive problem which makes it unattractive to 99.9% of UK GA pilots.
It costs money! A Brit will never pay for something which he can get for nothing, simply by wasting half an hour hanging around the airport doing it manually using some terminal in the briefing room. If there is a briefing room.
Euro 4 per flight plan filed, approx. This is so much more than the pilot spends on avgas on the flight he is filing.
But seriously, a "simple web page" (with no login security) was never an option because any old moron could have used it to spam the system with thousands of flight plans, starting with a Concorde flight from Goodwood to Bembridge and then moving onto more serious stuff. This is where a bit of imagination comes in: if you validate a list of permitted tail numbers, you avoid the need for security completely.