Your individual IHT nil rate band (tax free) is the same whether you are married or unmarried.
It is £325k + an extra £175k if you leave your main residence to your descendant as described above.
You can either leave all your assets direct to your descendants, in which case your estate will be liable for IHT on anything over £500k if it is worth more than £500k ( which may be the case now that a pension pot will be included).
If you are married you can leave assets to your spouse (or CP) with no IHT at all due at that point.
Then they inherit your £500k tax free amount to add to theirs if their own will leaves the property to direct descendants, and at that point their estate pays the IHT on anything over a £1m including the assets they inherited from you.
So being married just leaves the IHT to be paid on anything unspent when the surviving spouse dies. It doesn’t protect the value of the estate left to your Dc in the long run.
Plus as a pp said, men especially are very prone to re-marrying quickly , usually to a younger woman and without sorting out the details of a new will. I have seen 3 of my friends watch their mothers share of the marital assets disappear into the hands of a second wife.
You can leave your own assets to your Dc whether married or not.
BUT it only works if the house is owned ‘in common’. You can sever a joint ownership if both agree, you need a solicitor.
And to be sure there is no legal challenge to the will you need to ensure that your spouse or a dependant (which could include a partner) is adequately provided for, I.e can live off their own resources or something left to them.
AND if leaving your share of the house to your Dc, whether married or not, you probably want a ‘life interest’ written into your will which allows them to continue living there even though it has been left to your Dc.
So you can see from all this that yelling ‘just get married’ doesn’t actually answer your needs if you would prefer to leave your assets direct to your Dc. You would still need a solicitor to deal with becoming Tenants in Common and writing specific clauses into your will.