One Shot Drain Cleaner - Instantly unblocks drains in just one shot! One Shot is the original and best drain cleaner available in the UK. One Shot has been sold throughout the UK for over 33 years. Tried, tested and trusted, the One Shot brand of drain cleaner has been used by generations of people to save money and keep their drains flowing.When poured into a clogged or blocked drain, One Shot instant drain cleaner goes to work instantly. dissolving grease, rags, soap, and paper towels. One Shot should be used as a preventative maintenance product on a regular monthly basis and you will not need expensive professional plumbing services.