The test hole would be more than likely for a Proposed ;
1 1st/2nd floor floor extension above the effected.
2 an adjacent and adjoining extension.
3 Investigation for Repair/Underpinning
Current Regulation is foundations at 1 metre deep or further subject to ground conditions, Drain Invert level and Tree Proximity.
All determined if an Extension is being added finally by the only man that can, the Building Inspector.
No addition of brick rubble or underlying Hardcore of a foundation is permissable, what has passed for some as usual practice in the anals of history is in the long and dark past.

That normal, when builders needs to fill up they use any sort of stuffs.

No this won't be a problem if it is just a random brick or a couple. This is a common thing i come across. Just take them out.

These random bricks are just debris in the trench back fill after building the walls when the ground was leveled. It would be far more unusual not to find any.

It shouldn’t be a problem providing the foundations are solid And the bricks aren't loose as the concrete would have enveloped the bricks and therefore they’ll now be part of the concrete.

Possible it’s waste materials from the original build how deep was the concrete

yes that is a problem as it seems like it's been built on made up ground and not on a proper footing