Is it OK to use pedestrian phase of pedestrian crossing to join/leave the carriageway? (UK) If the start of my journey is on a busy road, and there is a pedestrian crossing nearby, I often take advantage of the pedestrian phase of the crossing by mounting the bike, joining the carriageway and setting off while the lights are red for traffic already on the carriageway. (Making sure, of course, to take care of pedestrians and other users of the crossing.) This seems to me a safe and convenient way to join a busy road.

Is this common practice? Is it advisable? Is it Legal? I don't consider it "running a red" as I'm already over the stop line by the time I mount my bike, but I could see how it could be construed that way.

Likewise if I'm fortunate enough to "catch a red light" at a pedestrian crossing near the end of my journey, I might take the opportunity to dismount and enter the crossing from the carriageway, cross to the footway and finish my journey as a pedestrian. Is this also advisable/safe/legal? In this case I am crossing the stop line under a red light, albeit "as a pedestrian".

If it makes a difference to any of the legalities, I live in the UK - Scotland to be precise.
I doubt the Police would stop you for doing so if you're not causing a nuisance or danger to pedestrians or other road users. I've seen it done plenty of times by cyclists and done it myself.

The Highway Code rules that apply to this are: 

Rule 64:
You MUST NOT cycle on a pavement.

Rule 79:
Do not ride across equestrian crossings, as they are for horse riders only. Do not ride across a pelican, puffin or zebra crossing. Dismount and wheel your cycle across.

At the start you should wheel your bike on to the road and start riding. If you're not on the crossing you are just taking advantage of stationary traffic.

At the end, you are allowed to get off your bike and wheel it across or onto the crossing. Pedestrians regularly enter crossings from the carriageway. Just make sure you have time to do so.