Don't know if they are still available from BMW or not, but here is the BMW part number for the touch-up 'pencil' paint code 050 : 51911240328.
Part number for an OEM spray can of paint code 050 : 51911240306.
Glasurit was the original supplier for paint, do a search, and see if you can source it locally in the UK.
Other than this, you may have to go to an automotive paint shop , and see if they can mix up an equivalent color, though cost is probably out of line with the amount of paint that you need.