Assuming all assets are in joint names, they each have an estate of £565,000.
I assume under their Wills all assets will be transferred to the surviving spouse on the death of the first spouse.
They each made gifts of £140,000 in January 2018. If one were to die tomorrow, the nil rate band of £325,000 will automatically be offset against these gifts. No IHT will be due.
Then the Estate of £565k will be transferred to the surviving spouse, no tax will be due on this due to spousal exemption.
The surviving spouse will have a nil rate band of £510,000 (at current rates, but may change), plus probably the residential nil rate band to use on their death.
It’s not very difficult but they should really get advice tailored to their actual position rather than relying on someone on the internet!! They aren’t short of cash clearly.