As pp have said look for local small family firms that have been going for decades. By local, you could be talking 30 miles depending on where you live.
yellow pages is a good place to start ( yell dot co dot uk ) bearing in mind the company might concentrate on double glazing products more than doors.
You may also want or need to replace surrounding side windows.
Does your house alarm have a sensor built into the existing door. Our old wooden door did and we were told we can’t drill holes into the upvc composite door / frame to accommodate the sensor. Don’t know if that’s true or the chap who installed the new frame and door just didn’t want the hassle/warranty worry.
Once you’ve been in touch with a company , ideally visited them to see what your gut reaction / instincts tell you about the firm, they’ll send someone out to discuss options, measure up, ask about payment - the best firms want a small deposit and payment when the door has been installed and you’re happy.
There might be a second visit before installation which is an exact technical measure and double check you are happy to proceed.
Bear in mind with upvc you can have the outside frame/door colour different to the inside, usually at an extra cost.
edited to add, we went from old draughty wooden door with glazing like the arch window in Play School plus a half height side glazing to a upvc foam filled door with small glazing in 3 vertical panes in the door, plus full height side glazing.
£1800 3 years ago. Live in North Wales.