For oil-based paints buy a Brushmate 4
http://www.decorating-supplies.co.uk/epages/BT3825.sf/en_GB/?ObjectPath=/Shops/BT3825/Products/110Stores up to 4 brushes, keeping them wet so you don't need to worry about them. Just don't forget what paint is on each one! Only about 12-15 quid. It comes with a filter soaked in a chemical which keeps the paint from going off, this lasts about 6 weeks, but I've got them to last 3-4 months by masking the lid of the box to keep it air tight.
When you do come to clean them, a quick soak in white spirit, followed by a heavy working of Fairy Liquid into the bristles to break down the oil in the paint, followed by warm water to wash it away. You usually need to repeat this process at least once until the bristles are squeeky clean. Then give the brush a good rinse under a tap to remove the suds completely. Cleaning is a lot easier with quality synthetic brushes.
For water based paints, just wash them out in water when wet. It takes seconds.
F&B Floor paint is pretty good, but if you want something super-durable, then I'd use Coovar floor paint, which is very good on wooden floors in particular. You'll get double the volume for your money too.