OK a few legal points....
In your will, you can name a guardian. This nomination is perfectly legally valid. If there is a surviving parent with parental responsibility, they will be jointly responsible along with the appointed guardian for the child. I am not a family lawyer, so I don't know what significance the role of the guardian would have, but the appointment stands. It is not the case, for example, that if mum nominates Mr A and dad nominates Mrs B, then mum dies, that mum's nomination fails because dad survives, and then Mrs B becomes guardian when dad dies. When mum dies, Mr A's appointment is valid. You would need to check out what his rights would be. I imagine they would be subordinate to those of dad, but that's just a guess.
Re: Inheritance Tax - there is a thing called spouse exemption. Any gift you leave to a spouse in your will, or which passes to them via survivorship (see below) is not liable to inheritance tax.
Re: joint bank accounts etc - it isn't easier to access a deceased spouse's account than a deceased unmarried partner's account - the issue is whether the account is held as joint tenants. Property held by two people as joint tenants passes outside the will by the law of survivorship. So, if you have a joint bank account, you can simply present the death certificate to the bank and they will scrub off the deceased's name and you can carry on using it as normal. This applies whether you were married to the deceased or they were your partner/brother/ friend/grandad or whatever. If it's a sole name bank account, its contents are part of the deceased's estate and the surviving spouse does not have an automatic right to access funds (although most banks will provide a cheque for funeral expenses before the estate starts to be administered and before a grant of representation has been issued - this is because funeral expenses are the first priority debt of the estate).
There are clear differences between the way married couples and cohabiting couples are treated when they split. If you are living with someone and are not married, you should take specific legal advice on what your position would be if you split or one of you died, particularly if you own property together. Be clear about whether your home is held as joint tenants or tenants in common, what you would get from a will (make a will!!!!) and whether the financial contributions you have been making to the household would be recognised.
Here endeth the lesson!