If fitting a deadlock I'd look at getting one with a thumbturn on the inside. You don't want to risk being locked in if someone accidentally deadlocks it when leaving. It might be a requirement of insurance due to fire regulations.
I had Banham in to fit four locks, two to each of our front and patio doors. They also installed hinge bolts. I now have one pass-through key to all locks. Took them a morning to do the work. Total cost £1200 or so but paid monthly interest free over a year. Highly recommended.
My place has only got a Yale lock at the moment so could do with adding a 5 lever deadlock or whatever it is that is required for insurance.
Any particular brand that is recommended and any recommendations for locksmiths in North London? Cheers