The Nikon P900 ($597USD) would be ok on mostly stationary subjects in good light (how often do you get that in the UK where you are?), however the sensor size is very small ~ mobile phone sized. Don't forget, there are the startup, zooming, AF, and shutdown delays to deal with, and not having to deal with this is where the DSLR's shine. People do get quite good results with the P900 with familiarity and within the limits of environmental lighting /movement conditions - you should check the dedicated thread(s) for more.
https://www.birdforum.net/showthread.php?t=370653
https://www.birdforum.net/showthread.php?t=371851
If you are going to photograph anything moving, or even flitting around, especially in less than brilliant light, then you will want something much more capable.
There is a great special on the recently discontinued Nikon DSLR D7200 ($697USD = 36% off)
https://www.birdforum.net/showthread.php?p=3820165#post3820165
It is by far the best bang for the buck DSLR whose sensor is over 13 times the area of the P900, and the resolution is 150% (24 vs 16MP) as well . What this means is that you will get much better low light performance, as well as far superior tracking performance. The D7200 also has a 1.3x in-camera crop mode which gives an extra fps (7 up from 6) and a 2x overall crop (ie. your 400mm lens effectively becomes an 800mm one at 15.4MP). I would place it well ahead of the D3200, or any of the D3xxx series, or D5xxx series.
The 18-55 kit lens you mentioned would be pretty useless for photographing birds. You are going to need a 400mm lens as a minimum. Without spending mega dollars, your best bets are going to be the:
Tamron 100-400 f4.5-f6.3 (~$620USD)
https://m.dpreview.com/products/tamron/lenses/tamron_100-400_4p6-6p3_di_vc_usd
Sigma 100-400 f5-f6.3 (~$620USD)
https://m.dpreview.com/products/sigma/lenses/sigma_100-400_5-6p3_dg_os_hsm
Sigma 150-600 f5-f6.3 ($884USD)
https://m.dpreview.com/products/sigma/lenses/sigma_150-600_5p0-6p3_dg_os_hsm_c
If you are more budget constrained, on the D7200, try the:
Nikon AF-P 70-300 f4.5-f5.6 ($597USD)
https://m.dpreview.com/products/nikon/lenses/nikon_af-p_70-300_4p5-5p6e_ed_vr
(or the DX version at a pinch for $397USD which is a little slower with softer edges).
https://m.dpreview.com/products/nikon/lenses/nikon_af-p_70-300_4p5-6p3_ed_vr
In general, lenses are not the place to try and economize money, and there will be far more joy if you invest here. Sometimes it seems you can never have a lens that is long enough ! :cat:
For a similar sort of total outlay, there is an excellent goldilocks type choice. It is a high quality bridge camera with a 1" sensor (one third the size of the APS-C sensor in the D7200, but still over four times the size of the sensor in the P900) that is pretty much the only bridge camera that can mix it with the DSLR's, lens and AF tracking wise, the:
Sony RX-10IV 24-600mm f2.4-f4 ($1598USD)
https://m.dpreview.com/products/sony/compacts/sony_dscrx10iv
Sometimes it's worth stretching the budget to avoid some limitations and frustrations. The suggestions I have given are a pretty good starting point for beginners which will give reasonable to excellent results in just about all situations. There are a few other options of course, but these may be for the more adventurous and experienced. Things such as the obsolete Nikon 1 V2, or the just released, but more expensive (~$1098USD) Sony a6500 Mirrorless. Lots of practice will help your results with whichever rig you go with.
Good luck !
Chosun :gh: