UPDATE
So, write after posting that I hadn't heard back from my MP, I received a letter in the post telling me that she had already written to UKBA for me (a few days previously). Then (just like magic), I received a missed Special Delivery envelope the next day (and it was the number I was looking for, my prepaid return envelope). Today I picked up my package at the sorting office!
Current married name passport, most recent maiden name passport and marriage certificate all returned. Now I just have to wait up to 7–10 working days (of course, this would be right before a four day bank holiday
) and I'll have my replacement BRC!
Now just to change the rest of the stuff I need to change into my married name!
Best of luck to every one else. I do really recommend writing to your MP!