I'd very much doubt there is any such thing. You couldn't force someone to stick to a will they had made previously...what if your husband and yourself make mirror wills and then you die, he remarries and has children? No court would disinherit that family in favour of an old will made with a deceased spouse.
I think my will also has something in it about DH and possibly other beneficiaries dying within 30 days of me. I assume that is in case of being in an accident and one dying instantly and the other lasting a few days. I wouldn't want everything to be inherited by DH and then pass on to his family rather than being split between our families in that case. I probably need to look at my will though. I think we drew them up before we had DC, but with them in there as hypotheticals.
I know in America you can place assets into an "irrevocable trust" which cannot be changed without the permission of the beneficiary. I'm not sure if we have something similar in the UK but it might be worth researching?
I'd never recommend making a mutual will. As pp have said, I think you're referring to a life interest trust which gives specified assets (usually the main home) to (usually) children, whilst allowing the surviving spouse to remain living in the property till their death or remarriage.