The letting agent offices are open in ordinary business hours, and one should not expect the letting agent being available later, especially because the move in may take some time.
Most letting agency business close at 17-17:30pm, some are open also later,
You certainly did not organise well the appointment for the move in, but the letting agent is the party supposed to act professionally, and I don't think they did.
For sure, saying that you would be escorted by the security was totally unacceptable.
Even if you were disappointed and you showed you were, a letting agent should never act rudely.
I therefore think that you should submit a complaint and claim some form of compensation for the inconvenience and distress.
If you had to spend the night at a hotel, I would ask the refund of the hotel's invoice.
I'm not sure your complaint will be upheld, but you may escalate it to the property ombudsman.
Not the best way to start a tenancy, but this already happened, unfortunately,
I hope this information is helpful and I wish you the best.
Please do not hesitate to contact JA again if you need help and you may ask to speak with me directly if you wish ("For TheLawLDN only").
I would appreciate it if you could provide a positive rating, if you wish so of course.
Thank you very much.