Most of the legal framework that limits this is contained within the Working Time Directive. Details here - look at section 1.3.
Unless you are a night worker your day can be potentially quite long before your employer falls foul of the Regs, so there's unlikley to be a legal impediment because the 48 hours average maximum are averaged over several weeks.
This doesn't mean that your employer is obliged to let you do this, though.
http://www.admin.ox.ac.uk/ps/managers/hours/wo rktime.shtml#3