My partner and I have just had our will made up by a solicitor in Tooley Street.
It was not as straightforward as what you describe and I must say that they did a good job.
They are Anthony Gold Solicitors, London Bridge Office.
If they are still too expensive to your liking I would have a look at the charity that writes wills for free during the month of November, and in return you donate whatever you can afford.
They are will aid.co.uk
Vickt offers some good advice. Use a solicitor rather than a will writer. Price difference isn't that great, but the resultant will will best suit your circumstances and will be written by a qualified solicitor rather than someone who has been on a short training course.
Various of the charities offer discounted will writing services, through firms that support them, if you are making any charitable donation as part of your will... I think Amnesty is one example. Shelter may be another.