The items appear to be one pounder high explosive shells. The QF 1 pounder (AKA Pom-Pom) was an early 37mm British autocannon. It was used by several countries initially as an infantry gun and later as a light anti-aircraft gun.
Why were these items buried in a garden?
There are a number of reasons why, however the most likely is burial for disposal.
When servicemen returned from 20th Century conflicts, it was not uncommon for them to bring home souvenirs. Surprisingly, soldiers well aware of the associated hazard occasionally brought back unexploded ordnance. One easy of means of disposal was burying them in the garden. Out of sight, out of mind.