These tax avoidance mechanisms are all legal, whether it be ISAs, tax relief on pension contributions, taper relief on CGT, Potentially Exempt Transfers with regards to inheritance tax and so on. Putting assets in an offshore trust may not be straightforward for a lot of people, and would take expensive advice to achieve, but then for most people it's not worth it, as their assets upon death are unlikely to breach the IHT threshold (currently £325k).
Whether these tax avoidance schemes are morally right is a completely different question from whether they are legal or not, and this point was made earlier. If they are not morally acceptable to society, then society should lobby to have these loopholes closed. Simplistically attacking people for making use of existing legal measures to not pay tax is missing the target IMO.