If you buy an electric, you’re going to need an amp – so there’s another £100 or so (at least). Or you could get a small effects box like a Korg Pandora or Pod/VAmp, which would mean you could plug in with headphones, have a whole host of amp models and effects, and maybe even a built-in drum machine. A drum machine is an excellent tool for all musicians, especially aspiring bass players. Unless you think you’ll be wanting to perform any time soon, I’d recommend the effects unit a nice pair of over-the-ear ‘phones.
Cheap, or even a lot of the more expensive, acoustics tend to lack any real volume or tone and the brass strings that they sound the nicest with are expensive and tend to become dead after just a couple of hours playing.
The bass is a great instrument and, like all instruments, can bring you a lot of pleasure (as well as frustration!). If you want to do it, then do. You’ll always be able to sell stuff on if you don’t get into it.