Be careful, if you buy a so called dormant company it could have debts.
Also the articles will have to describe its activities accurately which must come under one of a list of defined charitable purposes, and this will have to reflect what you want to do. It must also be for the benefit of some section of society which again will have to reflect the people who you are out to help.
Having said all of this, under the new rules articles can be changed now.
As daft as it sounds a new charitable company company can be formed for as little as £40 if you can draw up the articles yourself and file them with Companies House as paper copy. Company formation agents will quote silly money.
Also be advised that the law has changed, you cannot register with the Charites Commission until you can prove with bank statements a historic turnover of over £5000 in a single year.
My avatar is a poor likeness.