With regards to Stansted, yes it is the designated security airport for UK airspace so any plane that has a deemed security threat that will be diverted there. So yes they try and keep it down the list for diversions where possible.
The other thing is that where possible a carrier will try to divert to an airport it has an existing operation at. Obviously, it may not always have a choice but if there are several hours to go on the flight, it probably will. If a carrier diverts to somewhere it does not normally operate from, there are Significant additional costs and arranging needed in terms of agreeing simple things like organising ground crew to get people off the plane, handing luggage, et cetera. Having no ground staff at an airport is a real problem.
I had this a couple of years ago on a BA flight that was diverted to Stansted, before they started operating from there again - they only started summer last year, since pre-Covid. We sat on the plane for several hours as BA first tried to wait out the problem and get us back to Heathrow, In preference to the trouble of trying to sort out getting us off. In the end, they had to negotiate crew to operator the land bridge, sort the luggage, ensure customs and immigration were prepared for an unexpected large international flight, etc.
With the exception of BA, most of the big international airlines don't have any agreements with Stansted so will trying to avoid it.