I disagree with the principal of inheritance tax.
Inheritance tax unfortunately IS one of those taxes that is a punishment for doing the right thing and prioritising paying off a mortgage (no doubt making lots of sacrifices along the way to do so) so that you have a place to live in your old age - and/or a place to sell to fund your old age, if you need to.
The fact that there are super wealthy people in the world has nothing to do with the principal of inheritance tax.
Forgetting thresholds for the moment, to keep it simple...I think it is fair to pay a capital gains tax less what you have paid - as this the genuine benefit to you.
Example:
You buy a house for £200 000.00. You pay it off, plus interest, over 20 years at 5% interest, so in total it costs you £316 000 in after tax payments.
Over 20 years house prices increased by 200%, so your house is now worth £600 000.
Your capital gain is £400 000. But offset against how much you have paid for the house (with money you have already paid tax on), your total gain is actually £84 000
£84000 is what your beneficiaries should pay tax on, not 40% of the total size of your £600 000.00 estate, which is £240 000.