You only need to pay IHT if half of the property value plus any other assets (savings etc) adds up to more than £325,000.
Joint tenants means that house passes to the survivor rather than next of kin, if there is no will but does not protect from inheritance tax.
If you are joint tenants then the surviving partner inherits the house fully - but if there is not a will, then any other assets (belongings, money, investments, pensions etc) will pass to the next of kin (children, parents, siblings, uncles and aunts, cousins - in that order).
If you really don't want to get married or enter a civil partnership and want each other to inherit everything, make sure you have valid wills and power of attorney in place for health and finances so you can make decisions for each other and just accept that you will have to pay IHT on anything over the £325 000
the PoA thing is probably the most important - a lot of people get wills, but overlook the power of attorney thing - if you are unmarried without a power of attorney and something happens which means you do not have capacity, only next of kin will be able to make decisions (example of a worst case scenario being switching life support off / who can visit in hospital)