I am sorry that your children's bursaries will be reduced, I hope it is not by too much. If they are thriving at the school, and it sounds as though they are, do all you can to keep them there, it will be worth it.
Please don't resent Keir Starmer (or his wife), for their education. It is parents who decide where their child goes to school, as long as child is happy with the school. He was there on a full bursary so his parents couldn't have been all that well off, and, like your children, had academic merit. It was also a long time ago and this is now, he is 61, everyone is feeling the pinch to some degree.
Feeling as you do about Starmer's education is pointless, it only eats away at you so please give it up. I hope you don't share these feelings with your children, they will be no better off for knowing their mother harboured resentment against someone for a situation not within his control, umpteen years ago. We all surely want to bring our children up to be happy for the good fortune of others and never feel jealous.
Keir Starmer's parents had four children, they were ordinary. His father was a toolmaker and his mother a nurse but she had a very unpleasant chronic illness so life was not plain sailing for the family.
My only child had a bursary at a private school, which used to be grammar (my late husband went there). It wasn't a great deal, I can't remember the proportion as it was a long time ago, bur it helped.
Congratulations on having two clever kids, they will do you proud.