My take on things:
- You absolutely should not only mix with headphones. For starters, the stereo field is totally over-exaggerated
- IMO (and general opinion of pro-dudes) best results are from using both a good set of studio monitors in conjunction with some crappy speakers (think 80's panasonic boombox/cheap stereo - something with not much bass and not much highs, mostly used for checking the mix). Headphones are mainly for recording and checking the mix towards the end.
- My advice would be to wait, save up and try to double your budget. You obviously have some speakers already and you would not get much improvement with a £150 budget at all. Double it and you could just about get a pair of Adam A3x's which are very decent (and even excellent in a good room).
When it comes to monitors, you totally get what you pay for. Be wary of 2nd hand aswell, as the main problems that arise with speakers are mechanical (either the cone or the tweeter breaking), which are only easily dealt with by replacing the whole thing!
If you get some cheaper speakers now, you will no doubt then go on to improve you other gear, or at least improve your skills. It wont be too long before you start to realise that these speakers, while seemingly a big step up at the time you got them, are not that much of an improvement and you wish you'd waited a bit longer to get a proper pair!....thats speaking from experience