I did some research on this when i was active on my pet project, "getting off the ground in a Blenheim in CoD"
After reading through a copy of the pilot's operating handbook i found online, that's what i could gather:
1) Extra tanks were installed in the wings for more range.
2) This made the aircraft too heavy to safely take-off with bombs loaded.
3) Higher boost was needed.
4) Only the outer tanks got 100 octane fuel to assist in take-off, enabling use of +9 psi boost.
This can be easily gleaned from reading the checklists. It was not as much of a WEP setting, as it was simply a "take off when overloaded" power setting.
There were also other kinds of weirdness involved because only one pair of tanks had the ability to jettison fuel, the default inner tanks.
This meant that the potential for WEP was completely wasted: crews were instructed to cruise to the target on the outer 100-octance tanks and drain them completely before switching to the inner ones, since the outer ones couldn't be drained by jettisoning fuel in an emergency.
As such, the 100 octane fuel was effectively used during the probably safest part of the journey at cruise settings (or at least when some element of surprise still existed, during the inbound leg), not during the return from the target for escaping flak and fighters when it would mostly be needed.
Historical loadouts for short range missions were what we use when we fly it in multiplayer on the CoD map: just 55%-60% of fuel (that is the threshold between inner and outer tanks), with the engines limited at +5 psi boost.