You are not breaking the law on private property, but if you are on the road you need to be present if in charge of a running vehicle: www.highwaycode.info/rule/123

You are possibly invalidating your insurance. If you left your keys in the vehicle, then you would definitely be invalidating your insurance. With keyless ignition, it is a very grey area. The contract of insurance requires you to make sure your car is "secure". Having done that - and you have - it should be covered. The issue with keyless cars, are the devices that can be used to boost the signal from the car to your key. As the car is already running, you have done half of the work for a thief. There are various tricks that can be used to gain access to the vehicle whilst the engine is running while locked. This then becomes a very sketchy area. The best advise is to not do it, or sit in the car.