I used WRT firmware, loved it, but it's certainly not for average joe. I am willing to bet what BT aka British Telecom will have to roll in their grave before allowing such tinkering with their equipment.
My best bet for OP is - get
https://play.google.com/store/apps/details?id=com.farproc.wifi.analyzer or similar app to scan for your Wifi networks space. Do notice if certain channels are less powerful than others. Also despite this will lower your max speeds, you might have MUCH more stable connection with narrow - 20Mhz channel in smack in the middle of very busy 1,6 and 11 channels. Lets say channel 3 at 20mhz narrow bandwidth and you'll have nice steady signal.
Also read this article:
http://arstechnica.com/gadgets/2015/10/review-ubiquiti-unifi-made-me-realize-how-terrible-consumer-wi-fi-gear-is/
My solution is to use pair of Asus RT66 routers. One in basement as a router and second on 2nd floor as wifi repeater - it would not bad. I had also great success with Apple Airport equipment for extending wifi networks.