It is important to carefully distinguish between Re Denley trusts on the one hand and true private purpose trusts on the other. Under a trusts law analysis there are two ways to look at this. One is a trust in favour of the UA which was made for the purpose of flowers/statute (Re Denley). The more traditional interpretation would be to see it as a trust for the upkeep of flowers/statutes which was made to benefit the UA.
The order matters. You cannot have a trust which has a private purpose as its object. What you can have is a trust in favour of individuals which has an underlying purpose. For example, I could declare a trust in favour of my nephew. I declare this trust because I want to pay for his university education. In reality, this is just a trust in favour of my nephew: once the money is paid it becomes his and he can use it however he likes. The bit about education is merely a statement of intent and is not binding.
This is what happens in a Re Denley-type trust. Re Denley trusts are not purpose trusts. They are simply trusts in favour of a specified group of individuals. Don't be fooled by use of the word "purpose" here: this isn't a purpose trust, it is simply the reason why the settlor created the trust and that is not binding. It is the same as the trust in favour of my nephew. The trustees - i.e. the committee members which control the bank account - may use the money in favour of the members of the UA and do not have to use it on flowers or statutes. The "purpose" is not relevant and it is not enforceable, a better word might be "motivation": once the trust is constituted the settlor's "motivation" is not relevant.
Purpose trusts are something quite different. If you see the "flowers/statutes" bit as being the most important, then the flowers/statutes are what was intended and the fact that it was intended to benefit the members of the UA is secondary. This kind of purpose trust would fail for lack of certainty of objects.
I think you should analyse all the different possibilities and explain what the consequences are. Then think about which possibility is more likely: is this a purpose trust intended to benefit the members of the UA, or a trust in favour of the members of the UA intended to be used for a particular purpose? In each case, only the underlined bit is important the second bit is secondary and effectively drops out.