Absolutely!
In fact you can even buy a kit from shops on the high street or even online for a mere £10.
If your wishes are simple and you want to leave everything to your spouse or children then a DIY Will is a viable option. You just need to bear in mind that in order to make it valid you need to ensure:
You are 18 or over.
You make it voluntarily.
You are of sound mind.
It's in writing.
You sign it in the presence of 2 witnesses who are both over 18 and not benefiting from your Will.
You have it signed by your 2 witnesses, in your presence.
If your wishes are more complex for instance, you have children with another partner, you're on your 2nd/3rd relationship, you're not married etc. then you are better off selecting a professional to do this for you. Take a look at "Who can write my Will?" to know your options.
If you choose to write your own Will take a look at DIY Wills to know what to avoid to ensure your Will carries out your wishes further down the line.