If this little lady was indeed rearing up to attack, she was having ideas waaaayy above her station. She is a harmless, missing sector orb weaver (Zygiella x-notata most likely). These are very common in and around homes, especially on windows, all over the country.
As you can see the head is light brown around the sides, a thin dark line around the side, with a thick, dark central stripe/chevron tapering back towards the abdomen. Steatodas (false widows) tend to have a more uniform colouring accross the whole of the head. The central marking on the abdomen is more like flare, or feather, Steatoda nobilis has a marking more like a shield or a pentangle. The spider in the photo has very clear stripes around the legs, adult noble false widows tend to have more uniform colouring on the legs, though sub-adult steatoda nobilis can have some striping.