I’m BA Silver until the end of next month and always find the fast track slow – always. And particularly rude. Hence I no longer fly from LHR if I can help it.
You can use the lounge at Kings Cross by merely enrolling in East Coast Trains loyalty programme.
I agree entirely about ‘ridiculous’ first-class full fares, which is why I don’t buy those tickets, in the same way I wouldn’t buy those airline tickets as they are cost prohibitive.