The plate does NOT have to be 100% vertical, the law states as close to vertical as practical.
Coppers have to get permission to do a second job, don't know about PCSO, Plastic Cop Specially Ordered, may not.
They do indeed have to show ANYONE with reasonable grounds for requiring it their warrant card, they also have to tell you their number and what station they are based at.
Might just have been yet another power tripper copper, maybe a probationer, as a lot are like this to start with.
tabetha