Is it only the inheritance tax advantage that can't be reached another way? Oh and I suppose the fact that they could change their will and you wouldn't know.
IHT is the main tax advantage that can't be accessed by cohabiting couples, there are some others
Married couples allowance (although this doesn’t apply if either spouse is a higher-rate taxpayer)
Transfer of assets free of capital gains tax
Bereavement allowance
ISAs after death:
While ISAs can’t be transferred between spouses during their lifetime, they can be transferred on death to the surviving spouse while retaining their tax-free status.
Pensions and bereavement allowances:
Unlike married couples, unmarried couples who live together are not entitled to receive the state pension or bereavement allowance for deceased partners.
If you have a defined contribution pension, you can name your partner as a beneficiary and they will inherit your pension when you die, whether you’re married or not.
However, if you or your partner has a defined benefit pension - most common in the public sector or large corporations - there are specific benefits that can usually only be received by a widow or dependent child. If you’re married, the surviving spouse can receive a ‘survivor’s pension’, sometimes for the rest of their life. An unmarried partner is unlikely to be entitled to this, although it depends on the pension scheme rules.
Joint bank accounts:
If you are unmarried and you have a joint bank account, on the death of one partner, the other partner becomes entitled to the balance and can continue to have unlimited access to the account. However, a proportion of the balance will be taken into account when calculating the value of the estate of the person who has died.
If a married couple has a joint bank account, the money is owned jointly regardless of who put it into the account. On the death of one partner, the whole account immediately becomes the property of the other
There are numerous other protections that can be accessed via a cohabitation agreement, with regard to financial arrangements in the event of the relationship breaking down, but this will cost several hundred pounds to be drawn up by a solicitor. This sets out exactly what assets each partner is bringing to the relationship and how they should be divided in the event of the relationship breaking down. This includes your property, its contents, personal belongings and savings. It can also set out how much someone has contributed to the mortgage deposit and repayments.
And yes you are right about wills being changed: it's legally harder for an unmarried partner to challenge a will if their partner goes behind their back to disinherit them, than for a married partner
Of course, these are rights and responsibilities that couples may decide that aren't right for them - and thus they should absolutely free to cohabit without having the legal status of marriage enforced on them by virtue of living together. If you want to enter into a contract, you enter into that contract willingly and mindfully. But equally the lack of knowledge around 'common law' marriages leaves many unmarried partners vulnerable