Sounds awful. FWIW, I did EXT-NWI a couple of years ago, and was quite impressed by EXT, considering its size. There was a massive security queue, but it was all processed with remarkable efficiency. However, if you're headed to London, that's probably not so useful.

Personally, if I'm going down that way, I'll get the train to Truro, and pick up a hire car from the station there. It takes 4h30, and if you pre-book, you can get some very nice 1st class fares. When it works, it's a nice, very relaxing journey. Sadly, when FGW get it wrong, they do it with some style.