There have been a number of thefts of bikes recently from the University of Birmingham campus, including from a locked shed.
As a minimum in a city such as Birmingham it is essential to lock your bike to something solid and immovable (such as a properly installed Sheffield stand), using the best D-lock you can afford. This should be fixed as shown in the photo, or (if necessary) as shown but not passing around the seat tube. It's important to fill as much as possible of the area within the shackle with metal, to make it difficult to insert a bottle jack. Additional measures are advisable, depending on the bike. For example, a cable through the front wheel, or, better, a second D-lock, especially if the front wheel is expensive and/or held in place with a quick-release mechanism. Consider also a cable for the saddle; Brooks saddles in particular are attractive to thieves. Finally, take with you anything that is easily removed.
Consider also modifications to the bike. Do you really need quick release? Nuts give thieves more work to do, and require an extra tool. Dynamo lights also require tools for removal, so they can be left on the bike.
Don't think that having an old bike will protect you from theft. Some of the many local scrap merchants will happily take anything metal that isn't secure.