you need to use a website like bestflights.com.au or webjet.com (i think these are australian websites, i dont know what the UK equivalent sites are). just perform a search and it will bring up all of the available flights and prices. you will find that it will be cheaper if you have more stops along the way.
i also suggest that you get in contact with a travel agent and ask them to advise you when airlines are having sales. you can save lots of money this way. I managed to get return tickets to London from Aus for only $600 (i think thats about 350 pounds) when singapore airlines was having a sale.
and the last thing you need to be aware of is when the 'peak' and 'off-peak' periods start and end (these are usually roughly based on seasons- summer/winter and special events xmas/easter). if you perform an internet search for flights on a particular date and it just so happens to be during peak time, it will cost you sooo much more. whereas, if you leave a bit earlier or later, you could save hundreds. Speak to a travel agent and they will be able to advise you when the 'off peak' dates are. flying during 'off-peak' season is much cheaper.