The most common and most likely answer to this crossword clue is the 5 plus 5 letter word MAJOR POWER
I've seen this crossword clue in The Guardian
M
A
J
O
R
P
O
W
E
R
Buddy explains!
The straight part
We know that "what Britain used to be" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what britain used to be = major power.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.