I have been in my current position for around a year now and my contract says I must give three months notice if I plan to leave, do I really have to work this?
Surely this is excessive and a bit discouraging to potential employers? I understand the company needs time to hire and train a replacement for me but can I not get out of this somehow if the period impacts on my career prospects?
This is in the UK by the way, just hoping someone else might have experience with this sort of thing.
do I really have to work this?
Unless you can negotiate a variation in your contract with your employer, yes, you do. It's a contract.
Surely this is excessive
Define "excessive". Putting it bluntly, if you thought it was excessive, why did you sign the contract? For what it's worth, three months notice is rapidly becoming the standard for knowledge workers in the UK.
and a bit discouraging to potential employers?
Yes. However, that's not your current employer's problem in any way, shape or form.
can I not get out of this somehow if the period impacts on my career prospects?
Almost certainly not unilaterally. UK courts will strike down completely unreasonable clauses, but three months notice almost certainly doesn't get anywhere near that bar. Again, if you thought this was going to be a problem for your career prospects, you shouldn't have signed the contract.
but can I not get out of this somehow if the period impacts on my career prospects?
In many cases you can, but you have to be smart and constructive about it. If both parties agree to separate, than it doesn't matter what the contract says.
In this case, your career impact doesn't matter: you need a concession from the company so you need to understand what's best for the company and how they can get it. Let's look at it from there perspective: They have someone who wants out but you can force them to work for you for another three months. That's not an ideal situation: while most people are honorable and will deliver decent work it's unrealistic to expect the same level of productivity you get from someone who is passionate about the job and their career at the place. The likelihood that the exiting candidate will go "above or beyond" is very low.
There is also the risk than an exit candidate will become contagious: while you are still there, you talk to your colleagues and inevitably your departure will come up. If the reasons are real and the new opportunity is attractive, many of them may get second thoughts as well.
So it's typically in their best interest to get you out of there as quickly as possible with the added benefit that they don't have to pay you for the full three months of reduced productivity.
So when the time comes: keep your eyes and ears open: how do departures went down for other people? How quickly do they backfill, do they backfill during the notice period or do they just wait until the existing person is gone. Based on this you can always suggest a solution: For example: "I'm offering to leave as soon as you want me to without pay for the notice period, if you reduce the period to 4 weeks".
