What is your source for this? Calling her "Lady Starmer" might make her sound affluent, but as far as I know, she became "Lady" when he was knighted for services to the Prosecution Service.
Obviously the Starmers are very comfortably off, but there is no evidence at all that they are multi-millionaires.
The most quoted source, here, doesn't even spell his name correctly and gives no evidence for the assertion he has a property portfolio of £7.7 million.
Sir Kier Starmer: How Much Is The Labour Leader Really Worth? – Prolific London
The Evening Standard - historically not afraid to stick the boot in to the Labour Party - describes the same land, bought for Starmer's disabled mother, as being on the market for £400,000 in 2022.
All change at Number 10: Inside Prime Minister Keir Starmer's property portfolio (msn.com)
If you have evidence to the contrary, do share it, but I think this is just another example of the discourse that they are all the same. Which they really aren't.