Count yourself lucky it has only happened twice to you - equipment changes on this route are very common (across all three LH planes). If losing the UD is very important, I suggest you change your dates - BA should accommodate you, especially if you leverage your social media connections.
There is virtually zero reason to email people about equipment changes - 95% of people who care are reading this forum! I suggest any involuntary seat change should trigger an automated email, which would cover off your situation.

Perhaps you should try out the alternative carriers on your next couple of trips in response, and see what you think of the service there?