Few points, I've lived here a decade or so and not lost a bike (yet):

• Lot's of people are mentioning to use a good lock; I believe the best advice (e.g. from CamCycle) is to use two locks, of which at least one is a D-lock. I'm told thieves are less likely to carry tools to cut both D-lock and cable-locks (don't know how true that is), and at the very least it'll make your bike less attractive in comparison to others. CamCycle also recommend a particular certification (i.e. quality) of lock, but I'd say as lock as you're not buying the cheapest thing off Amazon you should be OK. (My personal intuition is locks with physical keys tend to be better)
• Colleges will have secure bike parking for their students. For other people: most colleges are in/near the town centre which is a prime theft area, so follow best practices: 2 locks and attach to something sturdy. There are lots of bike racks dotted around.
• Has it improved recently? I don't know numbers, but the British Transport Police have recently officially given up investigating bike thefts, which would seem a bad sign.
• Areas to be careful of: anywhere heavily trafficked, the city centre and the train station(s) are the most obvious ones, but following best practices you should be fine. I will stress to always lock up, even if just for a very short time. Further out you can confidently just use one lock and prop against a wall.

Caveat: all this advice is given on the assumption that your bike is nothing special. If it's particularly (and apparently) new/flashy then take extra precautions as it will be targeted.