I fitted a tap onto my rain barrel today. You can find the parts at B&Q even if their suck-ass website makes it difficult to find the parts. Here are parts you will find there:
Outside tap (£5.99):
20mm wood spade bit (£7.48):
Drill a hole in your barrel with the spade bit at a height that makes it easy to get a watering can or bucket underneath it. If the platform your rainbarrel will rest on is high enough, you can drill a hole as low as you like.
Despite this being a non-threaded hole, you will be able to screw in the tap, forming a threaded hole on the way. You will screw it in clockwise, so wrap about 5 turns of plumber's tape anti-clockwise near the base of the screw. Screw in the tap. On the inside of the barrel, wrap more plumber's tape around the screw close to the barrel. Screw on the nut.
Keep the tap straight while you tighten the nut with a wrench. Done.
If you need to buy a 20 mm spade bit, contact me and I'll sell you mine. I bought one and drilled two holes with it and I'll never use it again.
This is how I did it and it works. If you can ask a plumber's advice, do it.