If you're hoping to, at some stage, have photos on here then you'll need a DSLR with a reasonable lens, so you're looking at Nikon, Canon, Sony and Pentax, all of which have a range of good cameras. There are perennial arguments about which of Canon or Nikon is best, but don't discount the others, they get good reviews and are aimed more at the amateur/consumer than the professional.
DSLRs and lenses can be expensive when you add it all up so a lot will depend on how much money you've got to spend.
For being reasonably close to the runway, with the planes that come to Southampton you'll need a lens, or lenses, with focal lengths covering the range from around 40mm or 50mm to 200mm.
You can check amazon for prices and see what's available.
For example there's the Nikon D3100 with 18-55 and 55-200 lenses that should be more than enough.
There will be similar with the Canon 550D, perhaps and either a 18-135 lens or 18-55 and 55-250 lens
You'll also need some SD
cards and maybe a spare battery.
For processing the pictures there's Photoshop (if you've lots of money) Photoshop Elements (which is enough for most editing purposes and you can buy from the Adobe website) or there's GIMP that's free and does much the same as Photoshop but without as much documentation.
If you just want to take photos then you can use a bridge camera or compact camera, but get one that has full control, i.e. not one that just has various scene modes. For this you won't want more than a x10 or so zoom. You need to be able to control the exposure rather than rely on the camera. Planes in the sky can confuse cameras because the sky is bright.
After all that, you'll spend a year or so getting to grips with the finer points of taking photos. It's not as easy as it looks and planes are challenging because (i) they move and (ii) the lighting can be difficult.