This article says the average home value is £187,000:
http://www.theguardian.com/money/2014/jun/03/uk-average-hous...
The tax is apparently 40%. So the heirs of a single person with a home worth twice the average will owe £11,600 in taxes (it looks like some of the exemption is transferable for couples, so ~0 taxes for lots of people).
It doesn't seem so onerous for people receiving a $600,000 house to have to pay $20,000 in taxes (it's certainly an amount that most owners of such houses will be able to plan for).