In reply to goldmember:
What the directive and the subsequent UK legislation allows you to do is to allow you, to clearly state how you think what you propose will effect the company and allow you to suggest mitigating suggestions of your own.
If you were allowed only to put your request in without having your say in how possible negative impacts will occur, the company could deny the request with several arguments of their own why they cannot be met.
This current system gives the claiment, you, who also has the biggest motivation for the request to be granted to come up with the best solution to any possible difficulties etc., that the company may face if someone wants to take time off.
So, if you have 4 days of work instead of 5, who is going to do the extra work? Are they likely to take on an extra part-time worker to cover the missing day? You will know whether this could benefit the company, perhaps because they sometimes face extra demands and someone whose working the one day extra a week may be willing to increase their hours, or whether taking on an extra body will simply be an additional administrative headache.
One of the benefits to the company of you only working 4 days would of course be a 20% saving on your salary.
So plenty of opportunity to put yourself in the position of your boss and tell him how this issue could be solved!!