You are of course right! They don't display the old forecasts, and its a pain in the arse!
I personally don't trust the web forecasts - either Met Office or BBC. The text invariably contradicts the pictures, and the overview pictures invariably contradict any animations! Oh, and the BBC site is often different to the Met Office site, despite them both having the same source data. I don't know if there is any forecaster intervention into what goes on the website, or if it goes straight from the computer (which is my suspicion). I also think they're just too general. I admit that I tend to use the BBC weather site out of sheer laziness, and I've not been impressed with the accuracy for a long time.
One suggestion would be to try the new area of the website:
Met Office : Invent - New Weather Visualisation
Not operational as such - still under trial, but certainly allows for more information to be displayed in hopefully a much more user friendly format.
However as I said before, the pressure charts do still seem to be accurate I think, which is why I've never really understood why other things can be so variable!
I do also think there are potential improvements on the way, but watch this space... I'm not convinced they have ever really completely got over the move from up here to the south west yet.
BUT if you are in a position to talk to a forecaster or read your own charts, I'd go for that any day