I got my A500 (new) for £90 from Richer Sounds in Newcastle, a couple of months ago
Maybe they've upped the price now (you don't mean the A5 amp is £120? - that's a different model). If it is the a500 they're selling you could always mention that it was cheaper up here, though, and see if they'll drop the price... It's a very nice amp, especially if you're planning to use it with other budget gear - seems quite forgiving in terms of sources used.
I didn't like the aural envelope speakers much - listened to them when I was buying speakers, and the midrange sounds a bit rough to me. Others do seem to like them, though, and they're cheap-ish - see if you can have a listen. If you're spending that kind of money on speakers you'll have quite a bit more choice secondhand as well.
Have a listen to some speakers with/without subs and see what you think. Certainly if you spend lots on floorstanders you can get good bass, and some of the cheaper ones have pretty strong bass too. If you're buying a sub and speakers I guess you'll have maybe £50 to spend on each - cheap subs may not have hugely deep or powerful bass anyway.
You can order most new stuff off Richer Sound's website to get delivered, but the secondhand and clearance stuff they sell you can't. I'd imagine you could lug an amp back on the train OK (I walked back up a good-sized hill with mine without a problem... but yeah, you wouldn't want to do that with floorstanders or a sub
If you listen to a few different speakers at a hi-fi shop you can at least get an idea of what you like - useful if you then go and buy it off e-bay...
Jon