http://www.britishairways.com/travel/home/public/en_gb
British Airways has a large global network offering over 600 destinations, along with their alliance partners.
http://www.easyjet.com/
Welcome to easyJet.com, the best place to book your cheap flights online. With low cost flights across many top European destinations you're bound to pick up a deal that fits your budget.
http://www.flymonarch.com
As well as being one of the UK's major providers of low-cost scheduled flights from bases at London Gatwick, London Luton, Manchester and Birmingham airports, Monarch is a key supplier of airliners to the tour operating industry.
http://www.flybe.com/
Flybe is one of Europe’s leading low fare airline’s operating over 152 routes to 56 destinations across 12 European countries.

http://www.cheapflights.co.uk
Cheapflights is a travel price comparison website.  With more than ten years of experience they are able to offer excellent deals on flights all over the world.