Europcar used to have a special offer called 'Boomerang' which is 2 * one-way rentals (at the beginning and end of your hol) for £69 I think.
Can't see it on the website at the mo, but there is another offer - half price one-way rentals to London. Not relevant to you, because the dates/ airports don't match, but worth keeping an eye on.
Also try
www.bcvr.co.uk, which seems to beat Europcar at their own rates.