1 If you are not familiar with HMRC R85 and R40 forms they're website will explain.
2 ISAs are a bit of a guessing game in your situation - in a perfect world they would yield the same as an equivalent taxable investment but they don't. The key is if you are going to be a tax payer in the future should you buy ISAs now as the annual allowance dies at the end of the year.
3 The return problem is relatively easy - follow MONEY above through to savings and adjust the calculator as required.