I pulled the following from
http://www.tisa.uk.com/regulations/42_ISARegs_2011B.pdfThe ISA must at all times be managed by an account manager and under terms agreed in a recorded form between the account manager and the account investor. SI 1998 No. 1870, Regulation 4(5).
The account investments must generally be in the beneficial ownership of the account investor. SI 1998 No. 1870, Regulation 6(a).
Except in relation to qualifying investments for a cash component, the title to account investments generally must be vested in the account manager or his nominee or jointly in one of them and the account investor. SI 1998 No. 1870, Regulation 4(6)(b).
In the non-cash ISA it would appear that the account manager takes title to the property and holds the property for the benefit of the account investor. This type of arrangement is similar to the concept of a trustee taking title to property and holding the property for the benefit of a beneficiary. This similarity may be where folks are getting the idea that an ISA might be considered a trust for U.S. tax purposes.
I have not worked through the regulation, but it does appear that the title to the cash in a cash only ISA may remain with the account investor. If the account investor retains title to the property in a cash only ISA, then this may be enough to prevent the arrangement from being a trust for U.S. tax purposes.