I would like to know the detailed rules concerning the purchase or
development of property in the different Channel Islands by a non
Channel Island resident. In particular I would like to know how it can
be done by individuals and also by companies.

The Channel Islands used to have a financial test and probably they
still do. They used to only allow new residents who have stacks of
money, where stacks = millions. BTW, they are not in the UK, hence
their financial advantages relative to the UK.