Very few thieves pick locks, or do anything at all sophisticated. Most break-ins are exactly that: the thieves break something: a door edge, a door panel, the door frame, a window. So, whilst a good lock is important, the physical strength of the lock and the keep, along with the strength of the door edge and frame is equally, if not more,important. And windows, especially those that are accessible but not well lit and visible, need to be secure.
Two-lever or three-lever locks are not good enough. They are easy enough even for a mildly sophisticated thief to open. A British standard (BS) 5-lever mortice deadlock should be the minimum on the outside doors. (Although it is possible to get a rim cylinder lock to an equal standard, it is more difficult and thus more expensive.)
But this page is about physical security.
Lock and keep fitting
It's difficult to make a mortice lock physically weak but botched installations are sadly not rare, so one would check that enough wood had been left around the mortice (the mortice is the hole a mortice lock sits in within the door body). One would also check that there were no splits in the wood — say from a previous attack. Do the same checks for the keep (also known as the strike); this is where the bolt engages with the frame. And check that there really is a metal, boxed keep; it's not unknown for a botcher to just make a hole in the wood of the frame and leave it at that. And a boxed keep, which is required by the British standard, is when there isn't just a metal plate with a hole in it; instead, the bolt receiver is a little steel box.
Rim cylinder locks need more careful inspection. Because they are fitted to the faces of the door and frame, rather than inside, they are intrinsically weaker. The screws, screw holes and surrounding wood should all be checked for strength and integrity.
There is more on all this on the improving security page.
Reinforcing a door and its frame
Because a mortice lock requires a hole in the body of the door, it creates a weak point. In a good quality door of adequate thickness (over 44 mm), there is a net gain. But if a door was really too narrow for a mortice, or is showing signs of weakness, perhaps having suffered an attack, reinforcing plates can be fitted. These are strong metal plates that look like oversized escutcheons with a keyhole. They are paired and bolted together through the door, but with the bolts not visible on the outside, naturally.
It's also worth considering whether your front door really is a front door and whether or not it's been installed properly. It's not unknown for conversions to leave flats with front doors that are only of internal door quality. An internal quality door might be hollow or filled with a paper concertina. It's also possible that a cheap door was used and fitted the wrong way around. In a cheap door only one side has the wood that's strong enough to take a mortice lock (this part of the door is called the lock block and would be marked when purchased).
If door edges or frame edges are weak, long strips of steel, called London bars and Birmingham bars can be fitted. In an attack , they spread the force away from the keeps and onto the whole frame. London bars have hoops in them so that they still can be fitted even if there is a rim cylinder keep (Yale-style keep) screwed to the frame. A Birmingham bar is a plain bar and can be fitted on the hinge side.
Also worth considering, are hinge bolts. These engage as the door closes, and they bolt the hinge side of the door into the frame. Birmingham bars and hinge bolts are even more important where a door opens outwards and thus has hinges that are more exposed and more at risk.
If you have glass panels in, or near, a door, deadlocks become even more important. Unlike simple latches, deadlocks cannot be opened even from the inside. (Only use them when you are out though; they are a fire hazzard if you are in.)
Bars can be considered for glass panels and windows. There are flat strips grilles that go on the inside and that don't look too obtrusive. The strips are approximately 15 mm wide and approximately 20 cm apart. We try to plan the grille such that the strip locations coincide with any muntins — the wooden strips that separate indiviual panes.
For windows traditional burglar bars can be considered.
We don't normally fit concertina grills. They tend to be more obtrusive. They are vulnerable at their lock. And they have been known to trap fingers as they are opened or closed.
Here is an example of the reveal fix kind of burglar bars we can install for you. (Reveal fixing is when the bar's fittings are within the window recess; face fixing is when they are on the wall around the window recess.)