I have made a few upvc doors safer on external buildings, the last one was very easy to see and get access to so i made a metal gate inside the door with a big lock on it that you closed last thing when locking up, if you have glass top and bottom in the door it could be useful to silicone the glass in which is useful if you have external beading, new high security locks are a good choice but the last time i had to get into a upvc door with no key it took me a few minutes and the owner was a bit shocked so an alarm might be a good move.
Okay, so in the end I decided against a steel door, I just don't have the spare cash at the moment, so I've modded my existing my door and frame.
Ive fitted a stronger 3 star euro lock barrel, a checker plate panel to the inside lower section of the door to help prevent it being kicked through (the door actually has a vertical section running down the middle so the lower panels were stronger than I thought anyway) and I'm currently fitting some 50mm angle to each side of the door frame to prevent the frame being crowbarred out (just need some coach bolts to finish off). The only external signs will be coach bolt heads on the frame so hopefully the door doesn't scream "expensive tools in here!!".
I've also made up some basic bars for the window, again on the inside as another layer of awkwardness. I'm not too concerned about the main door, it's a sectional up and over and when fitting it I never fitted the external handle, it can only be opened from the inside where it's bolted and padlocked.
Pics to follow once it all finished.
Basically if some scally wants to have a go at it now, they would have to make a whole lot of banging, crashing or grinding to get in.