The point about registered charities is that they are not intended to be formed for "one off" cases/ campaigns. They are akin to limited companies (in fact, the majority are also companies) and are intended to be ongoing concerns to support a given cause.
A registered charity is required to have a board of trustees, submit annual accounts and an annual return etc. to give funders some degree of comfort that the funds will not be misappropriated. Sadly, due to lack of audit trail, fraud in the charity sector, and especially the "unoffical" charity sector, is rife.
The Charities Commission is in a difficult place because there are literally thousands and thousands of small charities who don't do much/ are semi-dormant, but the administration of the governanace framework/ maintianing the register/ chasing paperwork, costs money, so you can understand why they want to make sure people are serious before they register them.
Agree with "outraged"that it makes it hard to get started. However, once you do, there are many forms of funding available to you that aren't available to everyone who thinks they have a cause, so on balance, I think the system is a good one.
Anyway, I dont think you should feel bad for not supporting every cause. As a funder, I spend my days saying no to good causes, especially very narrow ones, as it's not usually the most efficient way to solve the problem.