Any will which is properly signed and witnessed is valid. You don't need a will pack. You can simply write it on any piece of paper.
However a will solely determines what will happen to your estate. You can also specify what you would like to happen in regard to other matters but what you write is not legally binding.
For example you could state that you wish to be buried rather cremated. However, even if you'd actually paid for your burial in advance, there would be absolutely nothing to prevent your relatives having you cremated.
Similarly you can indicate that you'd like guardianship of your child to go to, say, your sister. But your sister would still be entitled to refuse, or the 'authorities' (i.e. Social Services) could still seek to make [what they might regard as] better alternative arrangements.
The 'Which?' guide, referred to by Dzug ('Wills and Probate' , available in most public libraries) is an excellent guide to writing your own will, without paying a penny. But, as Dzug states, don't expect a will to do things that it can't.
Chris