Few thoughts
1) Some prebuilt machines are cheaper than home built at present, mostly if you are putting higher end graphics cards into them because dataminers are gobbling up the graphics card stock (for bit coins and such until that bubble bursts). Now for photo editing you won't need a beast of a graphics card, but it might be something you read of in your research.
2)
Pick parts. Build your PC. Compare and share. - PCPartPicker United Kingdom
That site has regional websites so just change the country to suit your needs. Lets you pick out component and build a pc up from scratch and also has nifty links to most major online retailers worth considering. I used it when building as it makes it a lot easier to sort things out and share the build with others for advice
3) Data storage wise you ideally want
a) SSD (Solid State Drive) for the operating system (windows) and also for installing lightroom/photoshop and other software onto. SSD drives are much faster than regular harddrives and thus putting software onto them really helps with the loading speed of that software. They are not generally used for data storage as much because they cost more than regular harddrives for the same volume.
b) At least 2TB if not larger regular harddrives. These will be for storing your photos on.
With your budget chances are that you can not only afford several drives, but you could easily purchase and setup your own backup drives as well. So that your data is mirrored and thus reducing the chances of losing it. External drives are often advised for this as they are separate from your main computer and thus can be unplugged except when you are backing the data up.
Data backup is its own topic and there are a few ways you can do this from simply mirroring the drives to RAID setups.
4) When it comes to the case/tower that the computer goes into get a large/fullsize tower. It's going to sit on the floor under the desk anyway so larger won't be a problem (unless you live in a very tiny flat) and the other bonus is that a larger case not only has more room inside it for more harddrives and such; but also makes it a lot easier to put everything together.
Smaller cases can often be trickier because there's less room to move everything around inside.
5) Are there any other uses, besides photography, for the computer. In general a good end photography capable PC is going to run most of your bog-standard software without any issues; but if you are into music making, gaming, software work or other such hobbies/work/interests then you might have reason to consider a more powerful graphics card.