In answer to your question on tax, knowing what tax each person is liable for is incredibly complex and personal.
Here, however, is an extract from Section 3 (Interest & Fees) of the terms of the bonds that may help (and are related to the information shared by @Wickster as well)
Interest payments are made gross of tax. For Dozens bonds that you do not hold through an ISA, you will be responsible for paying any tax due on the interest earned.
Basic rate tax payers are generally able to benefit from £1,000 of tax-free income from savings (such as Dozens bonds), and higher rate tax payers can earn up to £500 of interest without paying tax. There is no allowance in the additional rate (45%) tax bracket. Otherwise please note that interest from the Dozens bonds is treated as (and therefore taxed as) income at your normal tax rates depending on your annual income level.
Any tax-free allowances must take into account all interest you earn across any financial products, not just Dozens bonds. Please note that we cannot give you financial or tax advice, and we are required to tell you by law that tax treatment (including ISA tax treatment) depends on your individual circumstances which could change in the future if your circumstances change.
There are no fee deductions from the 5% per year fixed interest rate
When you attempt to bid for a bond via an ISA there really should be more clarity about which tax year the purchase would relate to. Because I’ve checked via support, I know the relevant date is the date the bonds are issued. But given the lack of information in the app I don’t think it would be unreasonable for people to make an assumption that the bid date is the relevant date
I, unfortunately have just been rejected for the first time. A little bit slow off the mark because of the long weekend and then a series of work calls. Will be interesting to see the stats to determine whether it was the bid amount or just its tardiness which caused the failure.