He was acting suspiciously...
Which part of his actions was suspicious?
Filming on private property, when asked he left: Not suspicious (and note the police were not called at that point), and not illegal. If he had refused to leave he would have been guilty of trespass IIRC.
Taking photographs or filming the building: Entirely legal not grounds for suspicion.
Refusing to provide his name: Entirely legal, not grounds for suspicion
Refusing to show his images: Entirely legal, not grounds for suspicion.
If you can show me anything he did that was either illegal or grounds for suspicion sufficient to warrant detention then I will concede the point. You can't, because there wern't. His "crime" was lack of co-operation, again entirely legal.
If someone was walking down the road with a holdall stuffed with cash that was poking out the zip, he would get questioned about it.
He would be approached. He would be perfectly within his rights to decline to answer questions or provide his name. There is nothing illegal about walking around with a sack full of cash. A question about reasonable suspicion of robbery would be based on the circumstances.
If he had a good excuse/reason, fine, if not, you would expect it to go further.
I would expect that the man was within his rights to decline to answer any questions, and the officer would have to have reasonable grounds for suspicion that the man had commited a crime. In the case of a man carrying a huge holdall full of cash down the street it is a rare enough occurance, out of the ordinary, and as such arguably is grounds for suspicion.
In the case of a man taking photographs it is completely the opposite, it is entirely ordinary. Taking photos is not grounds for suspicion of terrorism, and the police's own guidance makes that clear.
Yes this isn't -as- suspicious as that, but filming on private property
Which he stopped doing when asked to, and at which time the police were not called. It is not illegal to film on private property, but you commit trespass if you do not stop when asked, and you probably commit trespass if you know it is not permitted and do so anyway. He was detained under section 44 which makes that irrelevant, and as noted the police were not called for that incident.
and being so secretive about his reasons
He is legally entitled to silence, a position that does not affect his rights in the slightest and does not provide grounds for suspicion.
makes him suspicous enough in my eyes at least, to make it worth them checking his camera.
I disagree entirely.