As Piglet says test the speed with a wired connection to the router first. If you get 8mpbs wired then your internet isn't great but it'll do. The issue with the speed to your house is determined by the distance of your house to the exchange.
If you are far from the exchange then nothing can be done to improve ADSL speed. It's just a fact of signal decay on a copper wire.
If your area is served by Fibre then your best bet is to switch to a fibre plan; however fibre is still affected by distance from the exchange unless you go with hyperoptic or virgin media who both do fibre to the premise.
If the signal to your router is good and it's then a degraded wifi signal around your house there are a number of options.
A) put in a switch and then run cat6 Ethernet cables around the house (as piglet said they're with RJ45 connectors). The wires can be purchased on a roll but then they don't have any plugs. A crimper doesn't cost a lot but is fiddly to use. You can purchase them in fixed lengths with the plugs already on.
B) signal booster/signal repeater. You install these at strategic locations around your house and they take the wifi signal and repeat it. They can be connected to the original router either by a cable or wifi signal. Rough cost of ~150 for the whole house to be well covered.
C) power line adaptors if you have the sockets available near your router. These funky devices turn your electrical circuits into data cables and can transmit reliably at about 300mbps. They're sold in pairs and require their own socket each. They used to have to be on the same ring main, but I don't believe they do anymore.
D) Finally, if it is a REALLY significant problem you can get a fixed line installed but the cost is significant- around £200/month. This will get you good service- about 40-50mbps but the cost is very very high.
Test your speed at the router (plug your laptop in or do the Speedtest with the laptop right next to the router) and tell us what download and upload you get.
Also need to know how many people are using it and what for, don't forget to include TVs if you use Netflix or amazon movies or BBC iPlayer etc. Also how old is your house and when was it last rewired?