You should get your hands on 100 pence so you can say
"Honey, I shrunk the quid."

*forehead slap*

ugh... oofff...

I'm not sure inflaction works that way...

what happens to the coin if you try to shrink it a second time?

The lumpier coin makes the force less even... So the coin gets ejected, and we can't find it. Someday someone with a metal detector is going to be really confused.

Have you considered trying to flatten out the lumpy one a little bit in an effort to even out the force before attempting to shrink it again?
what does that do to the properties of the metal? I guess it improved the compressive strength, but what about the tensile strength? or the shear strength. how about durability? are the coins more brittle?
so many questions.

It is just bending the metal... The volume stays constant (smaller diameter, increased thickness). Its abrupt, but more or less room temperature, it might work harden the metal but I don't think it has much effect metalurgically.

We did a peso (I think) four times, and nothing happened. It wasn't conductive enough.

We did an old arcade point token. It was really soft aluminum, and basicly turned into an almond. Its at work. I'll attempt to remember to bring it home and get a picture.

Is is possible for human made devices or procedures to change (reduce) the volume of the metal without loosing mass?

Metalurgy gets really weird really fast, and I don't honestly know. Different metal phases have different densities, and metals have several phases.