for the stansted area, depending on how much you want to spend:
REX. pretty weather, though it looks like you already have that.
uk2000 stansted xtreme as mentioned above. also worth checking the other xtreme airports they have there
VFR airfields vol 2. Includes about 80 or so local and major airfields in the middle of england. because stansted is close to the border between two products, you may want to consider getting vol 1 as well
VFR genX vol 2. Photoreal scenery for the same area covered by the airfields pack. Again may want to look at vol 1 as well. Downside is that it removes the default autogen so can look a bit balnd from some angles. The following packages really spruce it up:
Earthsimulations Treescapes. Accurate placement of autogen trees. Makes rural areas really spring to life. split into the same vol1/2/3 areas as the VFR packages, but i think its only something like 16quid for all three (i got them for a fiver off ebay...). It does have a fairly big hit on the FPS, but can be adjusted with the autogen slider
FSX power project 4.0. Freeware. Accurate coverage of global power plant chimneys, cooling towers, wind turbines, and the UK national grid pylons. Blends in quite well with the treescapes
UK ANO objects. Freeware. Mostly aerials and what not that are over 300 feet modelled and accurately placed. It does contain cooling towers that do clash with the power project ones, i would reccomend removing the files that start with CT (i think...) from this package.
UK refineries. Freeware. Quite detailled but again a little resource heavy. Basically it does what it says in the name - all UK oil refineries are included.
Aerosoft VFR londonX. VERY resource heavy. Need a pretty dam powerful pc for this one. Covers every building in a 120km2 zone of london. if you can run it well then its brilliant.
basically everything is covered there, except for a verson of treescapes that coveres accurate autogen buildings
If you want to try further afield, i would give earthsimulations other packages a whirl. I haven't tried them personally, but they look stunning. shawbury fields, alderney, scilly and guernsey in full 3d.