Some slightly dodgy advice on here. First thing to do is actually identify what the source of the issue is. Download a WiFi analyser app which will show you what other devices are broadcasting on a particular channel - you may find that there is significant interference which is causing the issue. First step would be to stand in each room and see which channel is most suitable - then force your router to use that and see if it helps. (there are typically 14 channels numbered 1-14 available to use)
Modern routers can operate over two frequency bands - 5ghz and 2.4ghz. 5ghz will give you faster speeds, 2.4 will give better range. Althiugh it should auto switch, you might find better results forcing it to use 2.4ghz only.
Using the analyzer app, you should be able to see the strength of signal as you walk around. Are there specific areas with bad signal due to things mentioned above such as metal in the walls?
Are you able to move the router to a more central location?
If none of the above fix it - and it genuinely is because you house is built like a bunker, then you need to find a different way to get signal into the other rooms. The best result is always wired - can you run an ethernet cable direct to the device you want to use?
If the router WiFi you have is crappy, you don't need a whole new router - you can just buy a WiFi access point from netgear, tplink etc for about £20 which is a dedicated device for the WiFi - which ethernet cables into your existing router /modem. You could therefore have it either in a different room, or perhaps a different area of the current room if it will help give better signal coverage across the house. These devices typically do a better job at WiFi coverage than the WiFi built into a router. (even the better ones which some isps like virgin or bt provide)
Only of there don't work, would I suggest going with WiFi extenders - they rarely provide good results and always take a hit on speed / stability.
Lastly - if you are paying for a big package like virgin media 600mbit fibre, you will never get this over WiFi anyway - you'll be limited to the speed you can actually connect on over WiFi which is likely to be a fraction of this. Mega fibre packages are more suitable to running many devices at once or using ethernet cabling to get the max speed.