What’s it got to do with lesbians? If it was a brother and sister, the same thing would apply, and would have done for many years. You may as well say a brother and sister should be treated the same as a married couple. It’s a bizarre argument to bring lesbians into it. How on earth, as the OP says, does this ‘discriminate against heterosexuals’?
Incidentally, the only reason their house will be liable for inheritance tax is the insane price of housing in this country nowadays. If house prices reverted to something approximating sanity, then unless they lived in a mansion, they would be nowhere near the inheritance tax ceiling. Not their fault, I suppose, but that’s the real problem here, not laws relating to lesbians.
Mike