I started out with sports photography 6 years ago now, nearly to the day. In that time, I've invested a lot of time and money into what first started as a bit of extra cash to stretch the student loan a bit further.
It wasn't particularly good at doing that, there's such fierce competition at the top level, and lower down it's very difficult to get anyone interested (I know Kipax has managed remarkably well with lower league stuff - but it's definitely not easy or for the faint hearted).
For starters, you need to be absolutely 100% confident of being able to get the picture. Not just any old picture, the picture that tells a story. You get good at that, then you might stand a chance of getting some sales or a %-split deal with a small agency.
Sounds great doesn't it?! It's not...I worked for a small agency, and when you're in the mix with everyone else, including the huge agencies, then you have to be very lucky to get a usage...so you've probably driven 100miles round trip and sat in the freezing cold for 90mins to come away with precisely £0.00
Even if you do manage to make a sale, the prices paid by newspapers are pitiful these days...after your 60/40 or 50/50 split, you'll probably see £30 per image. That would require 9 images per week just to make you £270, then factor in all the costs required to be a working tog (petrol, mobile phone contracts, professional body membership, parking costs etc etc) you'd need to be selling at least 12 images a week at your £30 per image.
Having been in the same situation, I can tell you now...that will not happen. Sorry if that sounds harsh, but it just wont.
After my attempts to make any money from this, I got so thoroughly fed up that I started approaching bigger agencies for work...the shift fees started out pretty bad. Again, you'd have to do at least 4 games a week to make your desired money...which again, will not happen. You also have to remember that not everyone gets a chance with a bigger agency. You need to have something to offer that no-one else can (ie. you live/work in an area that no-one else covers - such as me in Wales/South West)...you also need to be damn good at getting good photos consistently and quickly (ie. within 3 minutes for a goal, those photos should be on your agency servers). The guys who work for the big guys are the best sports photographers in the world...so competition is again, huge and fierce.
It's taken me 6 years to get to work with an agency that pays me respectable shift fees and expenses and offers me enough jobs that I can survive. I could almost give up doing any other photography work and concentrate solely on sport...if I was a "weekend warrior" like Andy, I could only now think about going full-time. That's after 6 years and a lot of money invested...much more than £12K i'm afraid.
Sorry if this comes off as a crushing blow to you...but it is very tough to survive in the sports world...there's a lot of guys out there who'll happily devalue the industry by working for peanuts, or conversely have so much control of the industry that no-one else can really survive.
All that aside, there are plenty of people on here who have shown that with time, money and a lot of effort and sacrifice you can (eventually) make it.