Off the top of my head: I think the motor have to be covered at all times so that random dirt cannot get in them (and can screw up the petrol car real bad), make sure everything is screwed tight etcetc when you race it (stress level on the cars are unbelievable unless you start to drive them), prepare to fork out a lot of money to upgrade and repair the car (every week, there is at least one crash which costs £5-10 pounds, it adds up fast). Especially for a petrol car, since that the car is full of metal from head to tail, a crash on a bad angle will cause lots of things to break(mostly due to shock), so make sure you are prepared to (financially) handle those.
For UK only, I think they use this site:
http://www.modelsport.co.uk/?gclid=CP7ygI73uaYCFcoe4QodAVe9Gw
But since that there are a lot of HK members (last time), they used to source their parts form Hong Kong since that its cheaper, and also from US (where parts are easier to find).
I took a look at your car, and I notice that the wheels are "jutting out". Please be careful when you crash, because I once crashed at 30-45 degree and the took out the whole front suspension (plenty of superglue, fancy glue and replacement to fix). That's with electric motor, no petrol, mind you.