Going to have to shop around for some options not available from OcUK, and also to penny pinch where possible.
Intel Pentium Gold G5400 3.7GHz (£47.99)
ASRock B360M-Pro4 motherboard (£64.99)
Patriot Viper Elite 2x4GB 2400 DDR4 memory (£77.94)
EVGA 450BT PSU (£28.49)
TeamGroup 240GB L5 LITE 3D SSD (£49.99)
R9 280 3Gb or GTX 680 2GB (£75 second-hand)
That's £344, and then a cheap case. Changing SSD to HDD would free up more pennies for case.
Also, for that budget, consider an older DDR3 system as well, DDR3 memory is cheaper than DDR4 and you can get hold of a little more CPU power.
i7-4770 (£115 second-hand)
Asus H81M-Plus Micro-ATX LGA 1150 motherboard (£45.95)
Patriot Viper 3 Black Mamba DDR3 1600 2x4GB (£55.99)
EVGA 450BT PSU (£28.49)
Seagate Barracuda 1TB HDD or WD Caviar Blue 1TB HDD (£35.80)
R9 280 3Gb or GTX 680 2GB (£75 second-hand)
£356 + a case. Or with i5-4570 second-hand it'd be £316 + a case.
I'm playing Fortnite on a custom 1920x1050 resolution no problem on a GTX 750 1GB + 4770K. On fullscreen 1920x1080 some textures wouldn't rez correctly (due to the low video memory), but windowed 1920x1050 fixed that. FPS is 80-ish when jumping out of the bus, and 70-ish on the ground for the most part with some dips to mid 50's here and there. So as far as handling Fortnite, don't worry, it's doable for £350 (without including Windows). Just a question of getting the best bang for buck and also thinking about any upgrades. If there's any major plans for the latter then I'd go with a DDR4 system even if slightly more expensive. But if it's just a new graphics card and/or an additional drive at some point, then not really an issue.