hey there I'm British too!
But before you buy please wait a while and find out a bit more otherwise salesmen will have you for breakfast. Please please please hold off spending your money! £700 will get you much better sound than your aiming for, however you have to spend A LOT of money before digital speakers make sense! That's because the really expensive ones are very different from the cheap ones, which are less about sound quality but ease of use.
In terms of value for money second hand is almost always better because hi-fi technology develops very slowly. Even dacs aren't made that quickly.
OK you CAN use the analogue out of your mac, but the sound isn't going to be nearly as good as an external DAC (it makes the signal digital to analogue). You're right that USB speakers would have their own analogue to digital stage, but you can't be assured it's going to be any good. Here's what I can recommend from personal experience:
A good system for you might be:
Beresford DAC
http://www.homehifi.co.uk/main/main.html £110 delivered
Second hand Kef reference 102 speakers with kube (the kube is important, its an outboard eq for them) -ebay £170
Second hand Quad 306 -ebay £110
Alternatively you could get the same speakers but with a
Cambridge Audio Azure 540r. - ebay £160
This would do the DAC and if you hook up extra speakers will do you 5.1 straight from your mac! I've owned this and it's a lovely amp, wouldn't be as good as the system above but much more plug and play and very expandible with LOADS of inputs.
That total system would set you back about £400, and sound really good. If you wanted to spend more money you could get an apogee duet instead of a beresford, the DAC in that is F***ing amazing. I've owned both (kept the duet!). If you want a sub, and you have to consider that using a sub may not necessarily give you better sound quality, it may just give you bass, the sensible suggestions are
REL quake
Rel Q100e
Rel storm
-All these are pretty cheap and available in the UK off ebay