I fly quite a bit internally, mostly for social events so I know the budgets airlines pretty well.

I'd recommend www.flybe.com

then also WWW.bmibaby.com

& http://www.aerarann.com/

to a lesser extent www.easyjet .com

and if you are stuck www.ryanair.com

If you are planning a few months ahead there is no reason why you cant get internal return flights from places like Birmingham, luton, belfast and the like for less than £50