If your I&E has been done correctly you will not be able to save much, apart from the allowances for car repairs and emergencies, to await those repairs and emergencies, as well as your share of overtime and extra income. Most IPs ( though a rare few have different ideas) will allow modest ( a few quid here and there) savings. But, if you amass too much, they will consider the allowances granted to be too generous and, possibly, amend this at review.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk