Quick googling came up with:
It is important to be aware, however, that for inheritance tax (“IHT“) purposes, HM Revenue and Customs (“HMRC“) treat each joint account holder as beneficially entitled to the proportion of the account which is attributable to their contributions. It follows, therefore, that if the deceased provided 100% of the funds to the account, then 100% of the balance in the account at the date of death would be treated as part of the deceased’s estate for IHT purposes.