Some British aircraft seem to rely on WEP a lot more than most aircraft. I think the Brits had lower continuous or long use boost settings than did most aircraft. The Mossie VI's 1 hour boost is +9lbs compared to WEP's +18lbs.
I would guess the Mossies were the same as the Spits / Hurris etc.
The 5 min hard limit in AH was IRL an 'advisory' limit.
After exceeding 5 mins WEP the pilots had to inform the techies so extra checks could be carried out.
I have yet to find an account of any Spit suffering engine damage or failure from exceeding the 5 mins advisory.
In one famous case a Spit pilot flew for OVER 30 MINs with WEP while evading the bad guys.
All due to Rolls Royce testing methods -
i) Take engine off the production line
ii) Run it to failure
iii) Find part that failed
iv) Redesign / strengthen part
v) incorporate into production engines
vi) Goto i)
They also tested a Merlin 130 at over 2600HP at their Derby works.