Hello, Den, and welcome to Talking Point. I'm sorry to hear about the situation with your parents, and that you needed to find your way here. There is a lot of experience here on TP.
Absolutely do not hesitate to call the police. By all reports, they are understanding when it comes to dealing with a PWD (person with dementia) and of course both of your parents need to be safe.
In fact, if your dad is a wanderer and/or you have concerns about him getting out of the house, getting lost, being safe on walkabout, and so forth, you can go ahead and contact the police (ring the non-emergency local number) and explain the situation to them. That way they can have his information (address, description, maybe a photo, family contact information) on hand should it ever be needed.
I would make sure your mum has a safe place she can go to in the house, where she can lock herself in, and to keep a charged mobile phone on her person at all times. That will provide some peace of mind.
If it's a choice between your mum letting him go out of the house on his own, and your mum getting hurt, then she might just let him go, and ring the police for help.
It can be very hard when you've got a wanderer or someone determined to "get out" as part of the dementia.
Others can better advise, and it depends on what sort of care package your dad currently has, and if his needs are being met, but perhaps a call to his social worker/social services to see about increasing his care? Maybe a day centre, or more carer visits? Or maybe some help around the house or garden for your mum? Possibly your mum could use some respite care, so she can have a break? Talk to the mental health team about a medication review to see if there is anything that can help your dad, medication wise? Get him checked out to make sure there isn't an infection or other problem making things worse, if this is a sudden behaviour change?
I also know that Age UK and the Alzheimer's Society have helplines that you or your mum can call, if that would be helpful.
Age UK
8 am to 7 pm every day, 365 days per year
0800 678 1174
Alzheimer's UK
Mon-Wed 9am-8pm; Thurs & Fri 9-5, Sat & Sun 10-4
0300 222 11 22
Very best wishes to you and your family.