Thanks for your question
As an unmarried couple you have two options as to the ownership of the property
1) Tenants in common
2) Joint Tenants
With the first - tenants in common - then you have the right to leave your "half" share of the property to whom you like, so children or your partner and make sure this is covered along with a declaration of trust, so do seek legal advise on this (with a solicitor who is familiar with property law) but it does protect the surviving partner from having to move and each of you own % of the property, and it also protects you from having to sell should either of you need to go into a care home.
The declaration of trust in essence ensures that the surviving partner is "lent" the half share for a set period of time. This also can avoid Inheritance tax, but you would need to take legal advise on this, as we are UK tax experts, so we can advise the tax position, but are not experts in the art of setting up and administration of trusts themselves.
With the second joint tenants, if either partner passes away, then their half share automatically passes to the surviving partner even if there is no will in place, as you are both treated as owning 100% of the property.
Either way, at each occasion Inheritance tax will have to be considered, as assets passed between you are not exempt as you are not married, and therefore for each of you, should your estate (including the half share of this property) exceed £325,000 then there is avoiding Inheritance tax, either on each other or for your children.
So this is why a trust (beneficiary) would be worth considering.
I am not sure what nominal tax you are making reference to, as the only tax position that would ever arise, is pre owned asset tax. And this only comes into play, when a property is gifted but the giftor remains having the benefit of the property, and this will not be the case. So for example you make the purchase and then gift a share each to your own children whilst alive and still benefit from the property. This would then require you to either pay market value rent (on the half share that the children then have inherited) or be subject to pre owned asset tax.
But this does not happen when one of the partners passes away and they then only own half the property as the other half have been bequeathed to surviving children of the deceased. This only happens if you gift your shares to your children whilst alive and continue to benefit from living there - and then DO NOT pay market rent.
You are merely considering putting in a right to remain or life interest within the arrangement, but this will not cost you anything should you be the surviving partner, nor would a trust if its written that no rents must be charged (as this could create a potential pit fall)
And clearly you want to take the tenancy in common route, its just a question of whether you wish to bind this in trust rather than have this written within the will, just make sure the rental income position is considered when just one of you survives, as this can avoid unpleasantness.
Finally the children who have inherited first (but have to wait due to the right to remain/life interest position) may find when the property is finally sold (when the surviving partner also passes away), may have capital gains tax arising, as they in essence will then be disposing of a half share of property that has not been their main residence that has increased in value, so do be mindful of this too.
Do feel free to ask any follow up questions. I have a meeting with a client shortly, but will be available from 2.30pm, and I will respond then, But if you could rate the level of service I have provided, it would be appreciated , as this ensures I am paid.