I'm working for a company in southern England having joined from university in the summer. I have worked above the level I was brought in at, developing projects, visiting clients, delivering solutions when I was brought in as support to then progress into this over a year. 6 months in I'm due my review. I was never expected to travel much for work but I do, and I work hard, I'm new I want to impress. Is it too early to ask for a pay increase? Considering the main thing here is, I'm doing a job above what I was brought in to do?
If you're doing work that goes above and beyond the expectations for the position when you were hired, you certainly have the right to ask for a pay increase. There may be better options, though, than focusing on asking for more money.
If you like the company and think you may have a future there, you could point out that you are doing work beyond your original expectations and ask that your review include a talk about your future career path, including aligning your title and responsibilities closer to the work that you're doing right now and adjusting salary accordingly. This lets the company know that you're not just interested in a pay rise; you want to manage your career over the long term, which is actually more important for a recent graduate.
On the other hand, you think you will move on relatively quickly, it may be more beneficial to ask for an adjustment to your title than to angle for a pay rise. Asking for a title change to better reflect your responsibilities will indicate your value to future employers and may increase your income over the long term more than a pay rise at your current title.
Bottom line - think about your long-term career path when review time comes around, rather than thinking only about short-term gain, and plan to focus the conversation accordingly.
Six months is pretty early to ask for a raise. A year to 1.5 years feels more reasonable -- especially if you are young / just out of university. You haven't "paid your dues" yet. Additionally, you may work hard and have a very bright future, but chances are you don't have a lot of polish (yet). Also, it is not unusual for there to be a moderate pay increase at the end of a 6 month probation anyway. Finally, they are paying you in experience that many don't get at your stage of career development. That will reap rewards later on.
However, if you feel strongly that you are grossly underpaid, or that your work is much more demanding than what your pay grade would suggest, and you must bring it up at 6 months, then I would take a middle-ground approach. I wouldn't demand a pay raise (yet), but I might bring up the topic.
At the end of the review I would say something like, "ok. Can I mention an important topic? I love my job and am happy here. However, since starting here I have taken on the additional roles of client support, traveling, project planning and development, etc. I'm interested to know if these additional responsibilities will be reflected in my compensation."
Then stop talking and let them reply. Personally, I don't think I would be any more aggressive than the above at 6 months. 18 months? Yes; you have a more proven track record. But 6 months? No.
This is a balance of your market worth, and your value to the company. Your level of effort and expectations are secondary.
If you are substantially under your market worth, and the company is happy with you--you may be able to lean it. You'll want to raise the issue gently so you can get a "no" and stay without tarnishing yourself. For instance, it's okay to ask out of curiosity if you'll still be in an annual review, or what the normal process is, if the annual review is used to give warnings, assign bonus/raises, etc. Ask without asking if you can.
If you are at your market value or your company is still unsure, asking is risky. It sounds like you are at your market value, and being patient may help to get a bigger raise when the normal process comes around.
(For completeness, if you're over market value or your company isn't happy, don't push the issue--pushing a decision could cost you the job. This doesn't sound like your case.)
Doing a job above what you were brought in to do is fine. Many promotions happen because people start taking over tasks above their pay grade. Consider this: imagine I told you that you have two choices. One--you can do the work above your pay grade for a year, and then there's a much better chance that you'll be promoted and recognized after that year with more money. Two--you can do the work at your level, and hope to be promoted and recognized next year based on the performance at the lower level. If you'd choose One, then you're right where you would be.