British author of primarily YA books 1970s/80s. His novels were very short and easy to read. To the left, he did a trilogy in which capitalistic aliens conquer earth but are forced to withdraw when the occupation costs them too much money. This author was killed in a traffic accident of some kind.
The closest I can find is Douglas Hill. He was Canadian not British but he moved to London and wrote most of his books there. He was killed (in London) when he was struck by a bus while crossing the road.
His writing is exactly what you describe as very short and easy to read, but then the books were targeted at young readers.
The trilogy you remember is the Colsec trilogy but you have misremembered it slightly. It is about a rebellion against an authoritarian regime on Earth but the regime is a human one not an alien one. The final book, Colsec rebellion, is on Google books and searching it for the word "losses" I find:
Coldly and logically, they were unwilling to add more losses to those which Colsec had already suffered. And so Galtrey had gone with the fleet prepared to begin talks with the rebel if the show of force did not panic them into giving up.
There is another trilogy, the Huntsman trilogy, that is set on an Earth occupied by aliens called the Slavers and time and memory may have conflated the two trilogies.
I need to give credit for this answer to Andrew Sawyer. He is the librarian of the Science Fiction Foundation Collection at the University of Liverpool Library, and Course Director of the MA in Science Fiction Studies offered by the School of English, and he is a very useful friend to have. It turns out he knew Douglas Hill and immediately identified him when I asked if he knew of a British SF author who had died in a traffic accident.