[...] So unfortunately I don't have space for a terrarium large enough to house a Nepenthes.
You don't need a huge terrarium. You could get away with a smallish fish tank. I have a x hookeriana (ampullaria x rafflesiana) in mine and it is no slouch. Whenever it gets too big for its boots the 'scissors of correction' come out. I think it is going all 'bonsai' on me because it now produces smaller pitchers but lots of them. The tank is not very tall and I would estimate the plant is 30cm or so including pot.
[...] So it does appear to be possible to create the correct temperature and humidity conditions for lowland nepenthes on a UK windowsill; using a thermostatically controlled soil warming cable and a plastic dome. However this is not a cheap solution. The soil warming cable cost around £30 and the thermostatic controller cost around £45. [...]
The tank above is 90x40cm (3' x 1' 4") which isn't very big (compared to some on here). If I were growing the Nep alone then I could get away with a much smaller tank (2' x 1') . The heat is provided by a 'reptile mat' and a controller (won the mat in an auction and the controller was about £10 (both e-bay)). The tank was my old fish tank that got cracked. Wouldn't trust it full of water but it serves just fine full of peat and stuff. The lighting is LED (again e-bay) and, apart from watering and 'pruning', I just leave the whole thing alone.
[...] But since my girlfriend moved in, the space I'm allowed to dedicate to carnivorous plants has been somewhat restricted ("houses are for people, not for carnivorous plants" she says).
Rebellion is the only answer. If houses are only for people then we can get rid of all that non-essential decoration, gadgetry, art, nick nacks, photo's, bathroom products, non-essential clothing and shoes ... children, pets etc .. need I go on?
Grab some stuff and make an environment that your pet amp. will like.