The accounts our DDs have are with the Coventry.
I don't know if that type of account is still available, though.
They are the beneficiary of the account, but it has an adult as the trustee.
The trustee can make withdrawals "for the benefit" of the child at any time. You would have to state what the money was for, or a cheque made out to the child would be provided (i.e. it can only be paid to them).
When they reach a certain age they can make withdrawals (14 I think) but they have to be countersigned by the trustee.
When they are 18, they can make withdrawals without a countersignatory.
The trustee can continue to make withdrawals as a sole signatory until the child is 18. This means, when they are 17 and a half, say, you can look at the situation, and if necessary, open another account in your child's name and move the money. You just don't tell them about the account!
I have no qualms about doing that if the situation warrants it. I'd tie it up in some kind of 5 year guaranteed bond, if needed.
Tax wise, it's tax free as the child is the beneficiary.