Estates worth over a certain amount at the date of death are subject to Inheritance Tax. The current threshold plus any allowance for your main residence are published on gov.uk.
If you believe your estate may be subject to Inheritance Tax and you want to see whether you can mitigate the amount of tax you pay you should consult a specialist traditional solicitor.
If you are an unmarried cohabiting couple you will not enjoy all of the protections of a married couple when it comes to inheritance tax mitigation.