Well given your budget of £400, and owning to needing a new case as well, then you have roughly £350 to get the rest of your components. The main sticking point presently is the cost of RAM, due to worldwide shortages of DRAM, memory prices have gone up significantly in the last year, and you can expect to pay around £140-150 for 16GB (2x 8GB) of DDR4 RAM, which doesn't leave you much room for the rest, so you'll be forced to settle for 8GB only for now. You've got one of two choices, buy 2x 4GB sticks and have the ability to run with dual channel memory, or buy a single 8GB stick with slightly less performance and add a second stick when you have the funds to do so.
So, buying 8GB is going to leave you with £270 for the board and CPU which is plenty, and lots of choice. I don't think there is a huge need to upgrade the graphics card for now, if you are running at 1080p/60Hz, the 780Ti performance sits between a GTX 1050Ti and A GTX 1060 3GB, so to get any upgrade you'd be looking at spending £250+.
If you don't need any fancy features, and RGB lighting etc. then I'd be looking at the cheapest board you can find that supports 4 RAM slots (if you chose to go 2x 4GB first), in the case of a choosing a Ryzen CPU, then you'd be looking at the £80ish B350 offerings from ASRock/Asus/MSI/Gigabyte none of them are terrible, none of them are particularly great, but they will all get job done, and allow you to overclock the CPU as well, should you want to. Again, taking the view of keeping it for three years+, then an R5 1600 (£180) is probably a good choice, as I would imagine that newer games coming out in the next few years will really start to make use of the 6c/12t CPU's and it may allow you to keep the system longer.
You could also try the Intel option if you so desire, it'll cost you more, £110 for the board, and £180 for the i5-8400, but may offer a slight edge in performance, in certain games, especially if you intend not to overclock the R5 1600. Obviously it will put you slightly over budget, and you won't get the full benefit of it with your current GPU, but should you get something as fast as a 1080 Ti in the next couple of years it would hold up slightly better (in today's titles) getting more FPS for you. Can't tell what it will fair like against a CPU with more available threads, but I doubt there will be much in it, and I am sure both options will satisfy the 60Hz requirement of your current screen.
Either choice you make won't be a bad one, and both options have draw backs and benefits, the next big upgrade cycle will be 2020 when you will have had this just over 2 years, and if you are still in to PC gaming, then maybe you might have a harder choice to make, with a bigger budget. For now, flip a coin, or just stick inside your budget, save some of the remainder for more RAM, or a nicer case (especially if you have to look at it), as long as you get to enjoy your games, I am sure you'll end up happy.
