DEWSNAP/ATTWOOD, Watford, 1920's
The basic way to trace your ancestors since 1837 is to go back generation by generation, buying the relevant birth and marriage certificates - and once you get back to 1901 make use of the census returns. It helps if you have an unusual name and Dewsnap is definitely uncommon - so following that line should not be too difficult. I note that you have already discovered the Dewsnap forum at http://genforum.genealogy.com/dewsnap/ and such forums can sometimes put you in touch with others researching the same family.
One good sign is that there was a Dewsnap family living in the Watford area at the time of the 1881 census. A 41 year old widow, Frances La. J. Dewsnap, born London, was living at London Road, Bushey with two daughters Augusta (14) and Florence (10) - both scholars born at Barnes, Surrey. There was also a general domestic servant Annie Griffin, 24, born Abbots Langley. Frances had income from dividends, and is also listed in the 1882 Kelly's Directory as living at Bushey Heath. Obviously you will need to bridge the gap between 1881/2 and 1921 - but the fact that the family were comparatively well to do means that they could occur in earlier directories in Herts or Surrey and that the late Mr Dewsnap almost certainly left a will which could be very helpful.
Let us know if you succeed in linking back to the 1881 family, or even earlier.
If you can add to the information given above tell me.