When teaching a PFL its difficult for the FI and Student to identify the same field, consequently its not supprising that some FIs chose one that is easily identifiable. Much better to pick a large square field!
Additionally, they may assume that as its an airfield it will be OK. Is it notified to DAP for inclusion on the UK Charts?
There is no exemption from Rule 5 when flying an approach and go arround at an unlicensed aerodrome, only when landing or taking off. So they should not breach the 500 ft rule but; that applies equally to microlights as well!