Bare minimum hardware wise you'll need...
(listed in order of priority for gaming use)
1. GPU
2. CPU
3. RAM
4. MB
5. HDD
6. PSU
7. Case
Note that only performance wise HDD and PSU are of equally importance, but the PSU is of very much importance for reliability and stability, so don't skimp on it.
Optional, software, and peripheral items are...
1. Display
2. DVD drive
3. OS
4. Keyboard
5. Mouse
Luxury performance items are...
1. SSD drive
2. Speakers
3. Headphones
4. UPS (uninterruptible power supply for storm prone ares with lots of power outages)
It's really hard to figure what level of parts you can use unless you tell us what items in the way of peripherals or OS you might already have. I can tell you though that 2000, even IF it's in GBP, can easily afford you a 1080 Ti if you just need the main parts without needing any peripherals, OS or accessories.
As for DVD drives, they aren't totally necessary for gaming anymore due to most games being available as digital downloads, but some games are still on retail discs, and such a drive also works to play music CDs, install drivers from disc, etc. Since you can get a good one for cheap, and your budget can easily fit it in, I recommend you get one just in case you end up needing it for some things, even if only infrequently.
As far as water cooling goes, if you don't even know what parts are needed to build a gaming PC, I would definitely not use a complex and high maintenance piece together custom loop setup. Also, since the AIO (all in one) factory assembled no maintenance water cooling units that outperform decent air coolers (and only slightly at that) can easily cost twice as much, I think it would be better to just get a descent air cooler if you need more than core parts. It will allow you to spend more money on the parts that do most of the work, like the GPU and CPU. It matters a lot though whether you plan to OC your CPU as to what cooler you choose. If you don't OC, there are many affordable air coolers that would suffice, and many of the better ones handle moderate OCs well too.