I would recommend either electric or propane. I personally use propane. It gives reasonably quick melts with no ashes to dispose. I have plans for an electric furnace but it seemed like a lot more work than my propane furnace.
Sorry I can't help with resources in UK but my burner is basically made from pipe fittings. It uses a TWECO fitting for the burner jet but you can make one from a brass pipe plug. The furnace shell is made from rolled sheet metal. The hardest thing to find may be the insulating material for the furnace.
Take a look at the propane furnace on my website. Also take a look at the furnaces on Cameron McKeown's site. http://home.iprimus.com.au/cmckeown/foundry.htm
Here is a link to plans for a small electric furnace. http://www.dansworkshop.com/Aluminum%20Foundry.shtml
There is also a Yahoo "HomeFoundry" group.