Thank you for the question. They will not be allowed to put an internal lock on the door as it will likely breach their insurances for fire purposes. However if someone has came into your room, I would report this (as they have requested) there is also nothing legally stopping your from blocking your door at night, just ensure you would be able to escape should there be a fire. I would report this to the landlord and if this is a serious matter they can potentially evict that person.
I hope this information proved helpful. You will find a local solicitor who deals with these matters on the law society webpage which is as follows;
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