Do you have a loft hatch door?

1. Create a new spare door with a large Xpelair ceiling fan fitted
into it - the very big ones are actually quite cheap (about £60 or so
for 300mm or something like that).
2. Fit a 5A socket near the loft hatch off the lighting final circuit.
3. Fit a battery powered heat alarm if you wish directly above in the
loft.
4. In summer fit the "fan loft hatch door", power up, done.

The heat alarm is in case the AC fan were to stall catch fire (Xpelair
will be EBM-Papst so will have a bimetallic strip cutout but this
cycles on & off so heat can build up quickly).

Realise house humidity will be pushed into the loft, which at night
will drop in temperature significantly - perhaps below the dew point
of that humid air causing condensation. Hence remove when the hottest
days of summer are gone.

Alternatively plumb the thing in properly with aluminium (or fire
rated) ducting to an outside vent in a gable end or soffit and a
backdraught shutter (otherwise it will bleed heat out when you do not
want it to such as winter). If the fan is quiet enough to run
overnight (you can get speed controllers) it will really make a
difference dropping downstairs temperatures.