It’s simply not true that all political organisations are secretive about their funding. Most declare their income and expenditure, and some give a break-down of income sources, including donors. The TPA does neither. It publishes abbreviated accounts which means income and expenditure are withheld. The last time it published full accounts was in 2006, when it recorded an income of £130,000. But the current organisation has ten full-time staff across two offices, which suggests either its income has jumped substantially or it is loaded with debt.
The centre-left campaign group, Compass, by contrast, publishes a great deal in its annual report (which can be readily downloaded from its publications page) - and goes beyond that which is statutorily required. Thus you can learn how much was earned through members’ subscriptions, members’ donations, grants from various sources, etc, and how that money was spent. And - “in the interests of transparency” - all donors who gave £5,000 or more are listed, along with the amount donated.
If Compass can do it, then the TaxPayers’ Alliance - which claims to stand for transparency and probity- can do it too. No, it is not legally obliged to do so, but the TPA should practise what it preaches.