There are many factors that affect your wifi signal. Cordless phones, security cameras, microwave cookers, fish tanks, lots of metal, distance from router, certain building materials, the amount of devices connected at the time...
I am still bothered about your upload speeds - they don't seem right. Virgin told me that their upload speed should be about 10% of the download speed and never much above 7 Mbps regardless of superfast broadband. Your's is much faster than your download speeds when you are not next to the router.