Inheritance tax allowances can be more favourable for married couples or those in a civil partnership than couples who are co-habiting - lets work through an example.
Tim and Jenny are in their early 50s, with two children aged 19 and 16. They are one of the estimated 3.6 million unmarried cohabiting couples in the UK. They have a mortgage-free house worth £500k, cash savings of £200k and £50k each in ISAs. They want everything to go to the survivor on first death, then to the children on 2nd death
First piece of advice would be make sure they have Wills in place. Anything not owned as joint tenants would pass via the rules of intestacy - and unmarried partners have no entitlement. They'd have to rely on a 1975 Act claim for 'reasonable financial provision'. Tim and Jen should also consider having Lasting Powers of Attorney arranged in case they can't look after their own affairs in the future.
Let's say Tim dies first. If everything passes to Jen there would be an IHT liability of £30k (possibly slightly less if the value of the half-share in the house is discounted due to co-ownership with a non-spouse). The unused residence nil rate band allowance of £175k would not be transferable on Tim's death to Jen. On Jen's subsequent death, anything over £500k would be taxed at 40%. An IHT bill of £108k would therefore arise and the children would inherit £662k.
If TIm and Y were married, there would be no IHT on the first death (X's) due to the spouse exemption. On the 2nd (Y's) death, the entire estate of £800k would pass tax-free to the children, thanks to transferrable nil rate bands. The children would be £138k better off.
Alternatively, if marriage is not for Tim and Jen, some planning could be put in place to include severing the joint tenancy and leaving an interest in the house on first death to the children in the Will - if not absolutely then via certain types of trust. This could achieve the same end tax result of no IHT for the children. If not done via Wills, then there is the potential for a deed of variation within 2 years of the first death to tidy things up tax-wise - although it's not a good idea to rely on this.
Of course, considering marriage for tax reasons is not entirely a romantic notion and some may not consider the future inheritance tax liabilities of their estate something to consider. It may be that life insurance to cover the potential IHT liability could be a route to go down.......and could perhaps be cheaper than a wedding!
However, whichever route you decide to take it's important to make a choice and put some planning in place - don't leave it to chance.