Absolute best solution is to run ethernet to the top floor, to a socket for hardwiring either your devices, or a wifi access point.
Second solution is a mesh WiFi - if you're on any of the BT Halo packages they will give you extender discs for your home hub for free, as a part of the whole home wifi promise, otherwise you can buy your own mesh wifi.
Option 3 - powerline - is great when it works, but depends very much on the wiring in your house. I tried this for a bit - it was flawless to some rooms, and wouldn't connect at all in others.
I avoid wifi extenders, they 'work' but have severe technical limitations, most notably that they reduce the bandwidth across the whole network in order to expand the coverage, which may not be a problem for some people, but in a house with lots of children using tablets, gaming, streaming it quickly becomes an issue .
I went the 'pro' route and put a ubiquiti network in that cost about £2.5k, but it is absolutely flawless in terms of performance and configurability - I have 3 gaming PCs, 4 consoles, 6 phones, 5 laptops, 7 tablets, 10 smart home appliances and 5 smart TVs all running with no issues. It also handles CCTV and runs a business web and email server.