Many UK citizens have an ISA which is a special account devised by the UK government, exempt from income tax and capital gains tax on investment returns. In other words, it allows you to shift wealth to avoid taxes. Is this unsavory?

Hate to keep repeating it but everybody arranges their affairs to minimize their tax bill thereby avoiding tax. It's not only legal, it's sensible. The government sets the rules (and changes or repeals them if it wishes) and good citizens obey them to the letter. If there's a highly devious suspect scheme afoot to minimize tax, the courts are there to examine it and make a judgement; if it is deemed to be tax evasion then serious consequences will and should follow.

Are those accounts tax exempt, or tax deferred? The equivalent in the US is called a 401k, which is just tax deferred. Still shifting wealth, so I suppose we're talking shades of gray.

Anyway, I'd just as soon do away with market-distorting tax code like tax-advantaged retirement accounts, mortgage interest deductions, etc and just lower the marginal rates accordingly.