I would try Etihad first - generally I have found Etihad the cheapest from London to Abu Dhabi - www.etihadairways.com and their service is on par with Emirates/BA and its a direct flight. Other than that try the usual travel site suspects STA/expedia etc - they may come up with a cheaper (though generally indirect) option. Other cheapish airlines to try are Gulf Air, Qatar Airways and Royal Brunei.
Also check Dubai as an alternative airport as its only around 1 hour 30 drive from Abu Dhabi.
One other possible cheap option ( I haven't checked the cost but a friend found it cheapest) is to use the Air Arabia (budget airline flying out of Sharjah) service to Istandul - get a cheap flight london-istanbul and pick up the budget airline from there - but perhaps alot of hassle for a few pounds saving....?
Let us know how you get on.