If you are going to get spacers get hub centric ones as they will be more stable. ECS tuning spacers worked fine no problems. Stay away from cheap eBay ones as they can fuse to your rotor and can be potentially unsafe.

Make sure you don't poke too much because;
1. Your wheel will start to rub on your arches and can potentially seriously damage them if your shocks compress enough.
2. Yes they are illegal and you might run into a cop that will give you a defect because your poke is too obvious.

Measure how much poke you will have with whatever spacing you get so that you don't get any surprises.

Police have much higher priorities than giving people defects for slightly spaced wheels, it's not like they will notice your wheels spaced by 10mm as you stated and pull you over.
But what about wheels with offsets that make them spaced you ask?
The regulations state that the wheel must not go out further than the arch, must be inside.

However, as stated before in this thread. Insurance will not cover anything if you have them installed so that's another risk you have to think about.