In case you're getting lost here, Sire, the general rule is that money held in a joint account automatically passes to the sole survivor upon the death of the other account holder. That occurs 'outside' of any will that the deceased person may have left. (i.e. it goes to the surviving account holder
before the will comes into play and therefore can't be left to anyone else, irrespective of what might have been written in the will).
However, as Peter P points out, there are circumstances where a court may rule that the general rule should not be allowed to apply:
https://www.gadlegal.co.uk/news/elder-law/the-inherent-dangers-of-joint-bank-accounts
With regard to Inheritance Tax though, half of the money (in most cases) in a joint account counts forms part of a deceased person's estate unless the account holders were spouses or civil partners. In certain circumstances though it's possible that all of the money in a joint account might count towards any liability for Inheritance Tax:
https://www.thisismoney.co.uk/money/wealthcheck/article-6565421/Is-money-held-mums-joint-bank-account-late-nan-subject-IHT.html