Good that you already have the router near the centre of the house. Using the WiFi scanner app can also help you identify weak or dead spots in the house, though, and if you're ambitious you can move the router around and use the app to see how doing so affects things.
Ethernet over Powerline doesn't require drilling holes. Basically, you buy a kit (containing two devices) and plug one in next to the router (not in a powerbar or surge suppressor), connect it to the router with a cable, and then in another room plug in the other device. You can then plug another cable between the second device and your laptop and get a connection to your router.
The data flows from your laptop (for example), through the cable to the second EoP device, and then through your mains wiring inside the house to the first EoP device, which sends the data to the router.
When it works, it can work well; unfortunately it's very much at the mercy of the quality of your house wiring - and what you've got plugged in. A washer, for example, tends to put a lot of noise on the wiring and can interfere with EoP operations. It is less prone to interference (v.s. WiFi) from a neighbour using EoP, especially in detached homes.
We use EoP in our home, and it generally works very well for us. In particular, my son - who plays online games - finds that the WiFi has lag issues that the EoP doesn't, despite the WiFi being strong and (unlike yours) not dropping.
As I said though, I don't know if it's even available in the UK, much less how well it works on typical UK domestic mains wiring.