There are other things to consider than just Inheritance Tax, although there are ways around this, such as setting up a Trust Fund.

 The trouble with DIY kits is the whole aspect of what happens is not explained well, inevitably you will not write the will to the effect that you desire.  If you serious about a will or getting advice on one contact this chap.

http://www.willplanningsolutions.co.uk/

Have you also considered power of attourney?  A will means nothing if you are still technically alive and/or critically ill.