For clarification, would he not pay tax on £500 of the interest in the joint account? (I'm following along for understanding, even though my husband and I keep everything strictly separate.)
Yes, he will pay tax on £500, and she will pay tax on £500 as well. In other words, as a couple, they will only pay tax on £1,000 instead of £2,000 if it was just held in one of their names.
The same approach will work if they held funds in a US or UK taxable investment account. When you sell shares you usually generate a capital gain which is taxable in the UK if you are resident in the UK. However, each person has a tax free allowance of £12,500 for capital gains, so if funds were held in a joint account then you have a combined allowance of £25,000 free from taxation on capital gains.
Each person also has a £2,000 tax free dividends allowance so if a couple has dividend income it is also beneficial to own joint accounts, or divide the dividend producing funds equally between them to maximize the tax free allowance.