Self-deprecation is a trait that permeates British culture. It is a national characteristic - evident in a sense of history that, possibly uniquely, dwells on 'glorious' failures (the Charge of the Light Brigade, Dunkirk, Scott's race against Amunsden).
The British have a horror of what they call 'blowing your own trumpet', and are deeply averse to earnestness, pomposity and self-importance. Statements that, in another culture, would simply be attributed as confident expressions of self-esteem, are misinterpreted in Britain as boastful and self-aggrandising.
If you want to avoid being misunderstood, learn to downplay your attributes and resort wherever possible to understatement. People will read between the lines and admire your modesty.